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).