Chapter 7 - Exponents and Exponential Functions - Chapter Review - 7-3 and 7-4 Multiplication Properties of Exponents - Page 465: 31

$b^{20}$

$(b^{-4})^?=b^{20}$ To raise a power to a power, we multiply the exponents. This means that the product of $-4$ and the second exponent must be equal to $20$. This is only true if the second exponent is $-5$ because $-4\times-5=20$ This gives us the equation $(b^{-4})^{-5}=b^{20}$