You can use seven. The key to this problem is that the total area is equal.

Total area of 3 ace 3a^2

The total area of 2b^2 of two B's

Total area of m c mab

The length and width of the final rectangle can be written as pa+qb.

When m = 7, 3a 2+2b 2+7ab = (3a+b) (a+2b).