Find the equation of the circle with centre $(-a,-b)$ and radius $\sqrt {a^2-b^2}$

Question

$\begin{array}{1 1}x^2 +y^2 +2ax+2by+2b^2 \\ x^2 +y^2 +2ax+2by+a^2-b^2 \\ x^2 +y^2 +2ax+2by-2b^2 \\x^2 +y^2 +2ax+2by-a^2+b^2 \end{array}$