What is the simplier form of the radical expression 3sqer125x^21?

I am guessing that you mean 3√ (125x^21). If not, forgive me.

So you know that 125 = 5 x 5 x 5

In this case, you have 25 x 5. You can square root the 25 to make 5. So now you have 3 x 5√ (5x^21).

x^21 = x^20 * x. Remember that √ (x^20) = x^10.

So you have 3 x 5 * x^10√ (5x) = 15x^10√ (5x).