What is a and b? Can you please explain what you are trying to do, what assumptions you have, and what data/constants you have?
You are mistaken about the square root function. For any positive number, a, there exist two numbers, x, such that . However, the square root is defined as the positive one. That is , , , etc.. -1, -2, -3 are NOT square roots.
(The reason we write the in "if then " is because the does NOT include both.)