This problem exercises the concept of "similar triangles."
With similar triangles, we can compare the sides using ratios.
The equation we can use to find side A is stated below.
Side 1A / Side 1B = Side 2A / Side 2B
The number refers to the triangle, and the letter refers to the side of each triangle. Side A in both triangles must be the base of the triangle, and side B in both triangles can be either of the other sides. The only restriction is that once we pick a side B on triangle 1, we must pick that same side on triangle 2.
A / 7 = 3 / 2
Then we can solve for the length of side A.
A = 7*3/2 = 21/2 = (E) None of These