I like to convert to exponential form and work with that
We can convert back to radical form at the last
Note that ∛ = exponent (1/3)
( 15x)^(1/3) * (9x^3)^(1/3) =
( 145 x^4)^(1/3)
Note ......(145)^(1/3) = (5 * 3^3 )^(1/3) = 5^(1/3) * (3^3)^(1/3) =
5^(1/3) * 3 (1)
Also.....note how to simplify ( x^4)^(1/3) = x^(4/3)
Make 4/3 a mixed number = 1 + 1/3
So we have x^(1 + 1/3) = x^(1) * (x)^(1/3) = x * (x)^(1/3) (2)
So...putting (1) and (2) together, we have
3 * x * (5)^(1/3) * (x)^)1/3) =
3x ∛ [ 5 x ]