To do this, divide each term in the parenthesis with 5b. When you divide exponents with the same base, just take their difference:
(5/5)ab^(1-1) - (10/5)b^(2-1) + (15/5)b^(1-1)c
ab^0 - 2b + 3b^0c
a - 2b +3c
I hope I was able to give a good explanation. Have a good day.