In order to solve the given substitution we just have to replace 2X - 3 for y into 4X - 3Y = 31, like this:
4X - 3Y = 31
4x - 3(2x - 3) = 31
4x - 6x + 9 = 31
-2x + 9 = 31
-2x + 9 - 9 = 31 - 9
-2x = 22
-2x/-2 = 22/-2
x = -11
Then, x equals -11. By replacing -11 for x into Y = 2X - 3, we get:
Y = 2X - 3
Y = 2(-11) - 3
Y = -22 - 3
y = -25
Then, the solution is (-11, -25) x equals -11 and Y equals -25
