So, then we combine like terms to get:
4x - y +3x +2y -21 = -2
7x + y -21 = -2
y = -7x +19
And finally put this in the third equation and solve for x.
So put in the first and second equations into the third one and solve for x.

So we have z = x+2y - 7 and we have y = -7x +19
SO in the third equation:
2x + 2y – z = 9
Wherver you see a z, put that equation, and where you see a y, put in the equation for y. (First put in the y in the z equation)