After adding 4 positive unit tiles to each side, we get:
3x - 4 + 4 = 2x + 5 + 4
3x = 2x + 9
which simplifies the original equation, because at the beginning there were independent terms at both sides of the equal sign and now all independent terms are on the right side (the +9 term). Notice that the other options don't give this result.
