If 5^x – 5^(x- 3) = 124*5^y, what is y in terms of x? GMAT Explanation

If 5^x – 5^(x- 3) = 124*5^y, what is y in terms of x?

A. x
B. x – 6
C. x – 3
D. 2x + 3
E. 2x + 6

Correct Answer: C