Answer
$b^2\sqrt [4] {56a^2b^2}$
Work Step by Step
Rewrite the radicand as the product of factors such that you can take the fourth root of certain factors:
$\sqrt [4] {56 • a^2 • b^8 • b^2}$
Take the fourth roots of terms to take them out from under the radical sign:
$b^2\sqrt [4] {56a^2b^2}$
