Answer
$$m = \frac{E}{c^{2}}$$
This is Albert Einstein's formula to describe his theory of relativity.
Work Step by Step
To solve for $m$, you need to isolate $m$ by itself. You do this by dividing by $c^{2}$ on both sides:
$$E = mc^{2}$$
$$\frac{E}{c^{2}} = m$$
$$m = \frac{E}{c^{2}}$$
This is Albert Einstein's formula to describe his theory of relativity.
