y = 13.5
x = 51/-6 -> (-8.5)
y = -3x - 12
-6x - 27 = 24
-6x = 51 first solve your first equation for x
x = 51/-6
y = -3(51/-6) - 12 substitute in your x value into the other equation
y = 13.5
make sure to check that your answers match up to make the equation true