-x - y = -8 → x + y = 8 → y = 8 - x (1)
x + 3y = 16 (2)
Sub (1) into (2)
x + 3( 8 - x ) = 16
x + 24 - 3x = 16
-2x + 24 = 16 subtract 24 from both sides
-2x = -8 divide by -2
x = 4
And using (1) ...... y = 8 - x → y = 8 - 4 y = 4
So { x, y } = { 4 , 4 }