One interpretation of your intent would be as follows:
16x^11y^8
Find 4 √ [-----------------]. Is that what you meant?
81x^7y^6
4*x^5*√x*y^4
If so, then the correct result would be 4 ---------------------
9*x^3*√x*y^3
and this could be reduced to:
4*x^5 * y^4 * √(x) 4*x^2*y
4 --------------------------- = -------------
9*x^3*y^3 √(x) 9
