Problem 3 - Entrance Test

Expand (7m + 3n)(7m - 3n).

A. 49m^2 + 9n^2B. 49m^2 - 9n^2C. 14m^2 - 6n^2D. 49m^2 - 21mn - 9n^2

Correct: B

Using the identity (a+b)(a-b) = a^2 - b^2, here a = 7m and b = 3n. So, (7m + 3n)(7m - 3n) = (7m)^2 - (3n)^2 = 49m^2 - 9n^2.