Show that f (2-3i) is a real number

It's not a real number. Remember that i represents √-1. The only way to make it a real number is to multiply i by itself (so i * i or i^2), which will make it -1

Distribute the f: 2f - (f*3i)

Since the i is still only to the power of 1, it is not a real number. It's imaginary