In order to solve this problem we must apply this simple rule:
√a.b.c = √a . √b . √c
So for that question we have:
³√125a6b³ = ³√125 . ³√a ⁶ . ³√b³
= 5 . a² . b
Let me explain now.
³√125 = 5 because we have 5 . 5 . 5 = 5³ = 125
³√a⁶ = a² because we can write a² . a² . a² = a⁶