Which expression is equivalent to b^-2/ab^-3?

The answer to this question would be: b/a
In this question, you have two variable ( a and b) with some exponent. To make the number express positive exponent, you can switch it location from denominator to numerator position, or vice versa. In this case, the calculation would be:

b^-2 / a^1 * b^-3=
b^3 / a^1 * b^2=
(b^3 /b^2) * (1/a^1)=
b^3-2 * 1/a^1=
b^1 /a^1=
b/a

1/ab^5 is your answer