Answer:
1/4 b^-3/2 - 1/2 b^(-5)
Explanation:
b^1/2 - 2 b^-3
------------------------
4b^2
Separating into two fractions
b^1/2 2 b^-3
---------- -- --------------
4b^2 4b^2
Remember when dividing exponents with the same base, we subtract them
1/4 b^(1/2 -2) - 2/4 b^(-3-2)
1/4 b^-3/2 - 1/2 b^(-5)