Wont let me add or subtract?

Read this, I cooked it up for you just now.

function Add(A, B) --Your function.
    -- Roblox is case sensitive so you have to use Capital A, and Capital B. Instead of a + b. SINCE you used Capital A & Capital B as variables when the funcion is called.
    print(A + B)
    return A + B
end

function Subtract(A, B)
    -- Roblox is case sensitive so you have to use Capital A, and Capital B. Instead of a - b. SINCE you used Capital A & Capital B as variables when the funcion is called.
    print(A - B)
    return A - B
end

Add(Variable1, Variable2)
Subtract(Variable1, Variable2)
Add(Subtract(53, 12), 12) -- Since your function uses return you could also do something like that :P.

All identifiers, properties, and keywords are case-sensitive. That means the capitalization counts.

a is not the same thing as A -- they are completely different variables!