(ax+by)2 = a2x2 + 2abx + b2y2 From the x2 and y2 terms it is clear that a = sqrt(36) = ± 6 and b = sqrt(25) = ±5.
to show that this is a perfect square, you have to show that -60 = 2ab This works for a = -6 b = 5 and a= 6 and b = -5. There are two solutions:
(6x - 5y)2 and (-6x + 5y)2