The second formula can be changed to X=7Z-2Y.

Enter the first formula.

28Z-8Y-3Y-6Z=0

22Z- 1 1Y=0

Y=2Z

Bring this formula into the first formula.

Then X=7Z-4Z.

X=3Z

So bring the required formula.

(45z^2+8z^2-z^2)/( 18z^2- 12z^2- 10z^2)

=(52Z^2)/(-4Z^2)

=- 13