hi, could someone show me the steps to answering this question please?

show that the lengths of the tangents from the point $\displaystyle (h, k)$ to the circle $\displaystyle x^2 + y^2 + 2fx + 2gy + c = 0$ are $\displaystyle \sqrt {h^2 + k^2 + 2fh + 2gk + c}$

thanks, Mark