You divide the coefficients and subtract the powers. You should get 4a^2

Exactly.

I'm assuming you mean (28*a^5)/(7*a^3)? If that's the case, here's the
answer: 4*(a^2).

Explanation:

First, divide 28 by 7, to get 4 in the numerator. Next, divide a^5 by a^3.
To do this, subtract 5 from 3 to get 2. Remember, 1/(a^3)= a^(-3). Then you
have (a^5)*(a^-3) in the numerator. To evaluate (a^5)*(a^-3), simply ADD the
exponents of the base,a, which is a^(5-3)= a^2. Since 28/7=4, the answer is
4*(a^2). I know it's confusing... This problem involves a mastery of the
properties of exponents. Good luck!