Question 552582
You take each factor and do a sqrt on it. Sometimes the factor ae perfect squares, but most times they are not. Many times the factors include something that IS a perfect square. So let's do the problem you noted in a detailed step by step way. Once you see the method, you will be able to skip some of the steps
{{{sqrt(252x^3y^4)}}}

First thing is to know you can break the factors into smaller peices. 
{{{sqrt(252) * sqrt(x^3) * sqrt(y^4)}}}

Now factor these smaller pieces with an eye on trying to find factors that are squares. Then do the square root on each of the smaller factors
{{{sqrt(36*7) * sqrt(x^2 * x) * sqrt(y^4)}}}
{{{sqrt(36) * sqrt(7) * sqrt(x^2) * sqrt(x) * sqrt(y^4)}}}
{{{6 * sqrt(7) * x * sqrt(x) * y^2}}}
{{{6xy^2*sqrt(7x)}}}