In order to solve this system by substitution, simply take the value of y in the first equation and use it in place of y in the second. This will allow you to solve for x.
x = -2y - 16
x = -2(-3x + 7) - 16
x = 6x - 14 - 16
x = 6x - 30
-5x = -30
x = 6
Now that we have the value of x, we can find the value of y.