Substitute y = x - 4 to the first equation:
-3x + 4(x - 4) = -20 use disrtibutive property
-3x + (4)(x) + (4)(-4) = -20
-3x + 4x - 16 = -20 add 16 to both sides
x = -4
Substitute the value of x yo the equation y = x - 4:
y = -4 - 4 = -8
Answer: (-4, -8)