7 (4.7 x 10^-3) x (5.2 x 10^-5)=

0.0000017108

Step-by-step explanation:

Well one example of why we need them is because we want polynomials to always the factor. In the real numbers, the polynomial x2+1 doesn't factor. There are no real roots of this polynomial. But the complex numbers are special in that no matter what polynomial you have, it will always factor in the complex numbers.

Good luck! :)