How do I make a script when the person is punched the other person gets knocked back?

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You could use a BodyVelocity.

local Debris = game:GetService("Debris")
local Event = script.Parent.PunchEvent
local Sp = 25

Event.OnServerEvent:Connect(function(Player)

	local Character = Player.Character
	local HRP = Character:FindFirstChild("HumanoidRootPart")

	Character.RightHand.Touched:Connect(function(hit)
	
	if hit and hit.Parent then
		local Humanoid = hit.Parent:FindFirstChild("Humanoid")
		local EnemyRootPart = hit.Parent:FindFirstChild("HumanoidRootPart")
		local BodyVelocity = Instance.new("BodyVelocity")

		if Humanoid then
			Humanoid:TakeDamage(25)
			BodyVelocity.MaxForce = Vector3.new(25000, 25000, 25000)
			BodyVelocity.Velocity = (HRP.CFrame.lookVector * Sp)
			BodyVelocity.Parent = EnemyRootPart

			wait(.5)

			Debris:AddItem(BodyVelocity, .2)
			end
		end
	end)
end)