For those that aren't savvy with Einstein, it's e=mc^2. The top five rows are connected to the squared. The rest of the equation is in the bottom five rows. The middle row contains all of the equation. The rest is up to your guesswork.
If you really want to make it easier, then make a 20x4 rectangle of Xs from the top-left corner, then a 3x4 square of Xs from the bottom-right corner.
If you really want to make it easier, then make a 20x4 rectangle of Xs from the top-left corner, then a 3x4 square of Xs from the bottom-right corner.