Answer
$\dfrac{a^{15}}{b^{20}}$
Work Step by Step
Use the rule $\left(\dfrac{a}{b}\right)^m =\dfrac{a^m}{b^m}$ to obtain:
$\left(\dfrac{a^3}{b^4}\right)^5=\dfrac{(a^3)^5}{(b^4)^5}$
Use the rule $(a^m)^n=a^{mn}$ to obtain:
$\dfrac{(a^3)^5}{(b^4)^5}=\dfrac{a^{3(5)}}{b^{4(5)}}=\dfrac{a^{15}}{b^{20}}$
