Question

There is one point in the function x + y = at which y = x. Drag the point on the coordinate plane to that point.

Answer 1

Set y = x and substitute into x + y = a:
x + x = a → 2x = a → x = a/2.
So y = a/2 as well. Place the point at (a/2, a/2) (the midpoint of (a,0) and (0,a)).