The right side has the apex of the roof on it, point (5,5). From the picture we see its slope is -1. It has to be the negation of the left slope for a symmetrical roof.
Point slope form tells us
y - b = m (x - a)
y - 5 = -1 (x - 5)
y = -x +5 + 5
y = -x + 10
Answer: y = -x + 10
