Do you mean this: $\displaystyle \sqrt{90*(2x-3y)^6}$?
Hints:
90 = 9*10
$\displaystyle (2x-3y)^{6}\;=\;[(2x-3y)^{3}]^{2}$
Let's see what you get.
Thank you. I only just joined the forums today. I've tried to get the math text functions down as best I could. The function I was trying to write appears as you have typed it. I'll take your hints and see what I can do with them.