y = 3x + 16, (1) x = 2y - 72. (2) Substitute the expression y = 3x + 16 of (1) into the equation (2). You will get a single equation for x: x = 2*(3x + 16) - 72. Simplify and solve it. x = 6x + 32 - 72, 5x = 40, ---> x == 8 ---> y = 3x + 16 = 3*8 + 16 = 40.