If you're asking us to solve for Z...
z² + 11z = -21/4
4z² + 44z + 21 = 0
Enter... *drumroll* the quadratic equation!
[tex]z = \frac{-b +- \sqrt{b^{2}-4ac}}{2a} [/tex]
(that's a plus or minus, sorry)
[tex]z = \frac{-44 +- \sqrt{44^{2}-4(4)(21)}}{2(4)} [/tex]
x = -0.5 or -10.5
Does this help? Is it too complicated? Do you have any questions?