Which could be the first step in solving the equation represented by the model below?

3x + 4 = 2x + 6

a. add 4 negative unit tiles to both sides.
b. add 4 negative unit tiles to the left side, and add 6 negative unit tiles to the right side.
c. add 4 positive unit tiles to both sides.
d. add 6 negative unit tiles to the left side, and add 4 negative unit tiles to the right side.