Verified answer

√(16 * x^4)

= √[(4)^2 * (x^2)^2]

= 4 * x^2

I'd say rewrite the 16 as 4^2.

Then rewrite the x^4 as (x^2)^2.

Then just square root as normal, getting an answer of 4*x^2.

√16x^4 =√16 x √x^4 = 4x^2