Since the denominator is the number on the bottom, we need to find a common denominator for a and 5a^2. Since a*5a is 5a^2, we have a common denominator - 5a^2! For the fractions, that means that since a*5a=5a^2, we have to multiply -7 by 5a to get -35a/5a^2-9/5a^2=(-35a-9)/5a^2
