You want to position LineHandle such that it connects two points. First for the sake of simplicity, it will make it much easier if the anchor points of all gui elements involved in the positioning of this line have their AnchorPoint set to 0.5 , 0.5. In addition, we're going to be using Vector2s for this problem as it's much easier to work with in this scenario.There are three things that need to be calculated: position, size, and rotation.

Position

Position is the easiest as it's literally just the middle of the two points. The position can be calculated by adding the two vectors up and then dividing it by two. This is exactly the same as finding the middle of two regular numbers because it uses the same logic. The position part would look like this:

--Position
local position = (PointOne + PointTwo) / 2

Size

Size can be calculated by finding the displacement vector between the two points and getting the magnitude of that. A displacement vector can be thought of as a line connecting two points (kind of like what we're doing right now). A displacement vector can be calculated by subtracting one vector from another. For example if you wanted to find the displacement between a vector called v1 and another one called v2, the displacement would be v2 - v1. The magnitude of a vector is its length. The formula for calculating magnitude is irrelevant here because it is already done for us and can be accessed by doing vector.Magnitude. By getting the magnitude of the displacement vector, we are essentially getting the distance between the two points (remember that we will be converting all our vectors in to UDim2s later).

--Size
local size = Vector2.new((PointOne - PointTwo).Magnitude , Thickness)

Rotation

This seems like the trickiest part because it requires a tad of trigonometry but it's not all that hard. First, we get the displacement vector again (explained in the prior section). In order to get our rotation, we will need to find the angle this vector makes with the x axis. If we think of this vector like the hypotenuse a triangle, we find that the x value of this vector is the adjacent side and the y value of this vector is the opposite side. In order to get our angle, we simply need to take the inverse tangent of opposite divided by adjacent or in this case, our y component divided by the x component of our vector. This is how it looks like:

--Rotation
local displacement = PointTwo - PointOne
local rotation = math.atan(direction.Y / direction.X)

Now, all that's left is to change the color of our line handle and convert our position and size into UDim2s. We also need to convert our rotation from radians into degrees. This is how the final function would look like:

function adjustLine(LineHandle , PointOne , PointTwo , Color , Thickness)

    --Position
    local position = (PointOne + PointTwo) / 2

    --Size
    local size = Vector2.new((PointOne - PointTwo).Magnitude , Thickness)

    --Rotation
    local displacement = PointTwo - PointOne
    local rotation = math.atan(direction.Y / direction.X)

    LineHandle.Size = UDim2.new(0 , size.X , 0 , size.Y)
    LineHandle.Position = UDim2.new(0 , position.X , 0 , position.Y)
    LineHandle.Rotation = math.deg(rotation)
    LineHandle.BackgroundColor3 = Color
end

Example Usage:

local line = ...
local frame1 = ...
local frame2 = ...

adjustLine(line , frame1.AbsolutePosition , frame2.AbsolutePosition , Color3.fromRGB(61, 128, 252) , 2)