The letters a and b are the real numbers and the letter i is the imaginary part. The letter i, when squared, equals as you know, -1. You can use the distributive rule to do away with the parentheses.
4i(4 + 9i) =
4i * 4 = 16i
4i * 9i = 36i^2
36i^2 = 36 * (-1) = -36
-36 + 16i is now in the form a + bi, where a = -36 and b = 16.