Not really understanding this, someone : )

Rearrange the product

(5.91 * 10^(-3)) * (8.7 * 10^10)

as

(5.91 * 8.7) * (10^(-3) * 10^10)

We have 5.91 * 8.7 = 51.417, and 10^(-3) * 10^10 = 10^(10 - 3) = 10^7, so the original product above reduces to

51.417 * 10^7

We can "add" powers of 10 by moving the decimal place to the left:

51.417 = 5.1417 * 10

so that

51.417 * 10^7 = (5.1417 * 10) * 10^7 = 5.1417 * 10^8

the answer is c. ⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀