Answer
$a = \pm \sqrt{c^2-b^2}$
Work Step by Step
Subtract $b^2$ to both side of the equation to obtain:
$$a^2+b^2-b^2=c^2-b^2
\\a^2=c^2-b^2$$
Take the square root of both sides of the equation to obtain:
$$\sqrt{a^2}=\pm \sqrt{c^2-b^2}
\\a = \pm \sqrt{c^2-b^2}$$
