Substitute y = 3x + 15 to the equation -4x + 7y = 20:
-4x + 7(3x + 15) = 20 use distributive property
-4x + (7)(3x) + (7)(15) = 20
-4x + 21x + 105 = 20 subtract 105 from both sides
17x = -85 divide both sides by 17
x = -5
Substitute the value of x to the equation y = 3x + 15:
y = 3(-5) + 15
y = -15 + 15
y = 0
