The very first thing you do ALWAYS is check out what values of x are impossible. The denominator is sqrt(16-x^2), so -4<x<4. This is quite simple math that just involves adding to both sides, multiplying, and factoring. Can you show us what you did so far?
**Hint** Get rid of the denominator to make the problem clean and simple. Doing that might solve more than one problem in the equation.