First, let's isolate one of the variables (preferably, y)
5y = -9x + 28
y = -1.8x + 5.6
Input this into the second equation:
5x + 9(-1.8x + 5.6) = 56
5x + -16.2x + 50.4 = 56
-11.2x + 50.4 = 56
-11.2x = 5.6
x = -0.5
Knowing this:
-2.5 + 9y = 56
9y = 58.5
y = 6.5
The solution set is (-0.5, 6.5)