simplifying radicals and fractional numbers 
$$\frac{a}{a-\sqrt{a^2-16}}$$ is the expression.
  I am not sure if i answered it right but please help me do this.


What you have is an expression, but not an equation. We can indeed simplify the expression.
We can multiply by the conjugate of the denominator, to obtain a difference of squares., e.g. $$\begin{align}\frac{a}{a-\sqrt{a^2-16}} & = \frac{a(a + \sqrt{a^2 - 16)}}{(a - \sqrt {a^2 - 16})(a + \sqrt{a^2 - 16})} \\ \\ & = \frac{a(a + \sqrt{a^2 - 16)}}{a^2 - (a^2 - 16)}\\ \\
& = \frac{{a(a + \sqrt{a^2 - 16)}}}{ 16}\end{align}$$
Added: What we've accomplished is called "rationalizing the denominator", which simply means getting the "radical" out of the denominator.
