I was working some mathematics and ended up with 4 unknowns with 4 equations. I tried substituting equations but ended up with squared unknowns as shown in the following picture:
I used Wolfram Mathematica to solve the problem numerically.. Just wanted to provide you with an answer. I don't have time to work it out on paper (at least for now).
You could try a technique called Completing the square if you want to solve it by hand (to the equations 5 and 6).