please answer it’s due tonight

A = first number (larger number)
b = second number (smaller number)

a = 8b
a - b = 70  *

* since a is the larger number and their difference is positive, we want a-b.

plug the first equation into the second to get : 

a - b = 70
(8b) - b = 70
8b - b = 70

Your correct answer would be choice #2.