what is the simplest form of square root of

First step is to simplify the inside of the radical. There are x's on both top and bottom of the fraction. So we subtract their exponents. x³ - x¹ = x². Because the x³ was in the denominator and it was bigger, the x² goes on the bottom.
Your new equation is:
[tex] \sqrt{\frac{126y^{5}}{32x^{2}} } [/tex]
Now, I'd suggest factoring each term to correspond with the square root.
126y⁵ = 9 · 14 · y² · y² · y
32x² = 4 · 4 · 2 · x²
9, y², and y² can be square rooted in the numerator. 4, 4, and x² in the denominator. So we pull out their square roots and place them outside of the radical. The rest stays in the radical:
[tex] \frac{3 * y * y}{2*2*x}\sqrt{\frac{14y}{2}} [/tex]
Simplify to get your final answer:
[tex] \frac{3y^{2}}{4x} \sqrt{7y} [/tex]
It's easier to explain on paper and in person than over computer text, sadly. Hopefully you can follow along with this.
[tex] \bf ~~~~~~~~~~~~\textit{negative exponents}
\\\\
a^{-n} \implies \cfrac{1}{a^n}
\qquad \qquad
\cfrac{1}{a^n}\implies a^{-n}
\qquad \qquad
a^n\implies \cfrac{1}{a^{-n}}
\\\\
------------------------------- [/tex]
[tex] \bf \sqrt{\cfrac{126xy^5}{32x^3}}\implies \sqrt{\cfrac{126}{32}\cdot \cfrac{xy^5}{x^3}}\implies \sqrt{\cfrac{63}{16}\cdot \cfrac{y^5}{x^3x^{-1}}}\implies \sqrt{\cfrac{63y^{4+1}}{16x^{3-1}}}
\\\\\\
\sqrt{\cfrac{(9\cdot 7)y^4y^1}{(4\cdot 4)x^2}}\implies \sqrt{\cfrac{3^2\cdot 7(y^2)^2y}{4^2x^2}}\implies \cfrac{\sqrt{3^2\cdot 7(y^2)^2y}}{\sqrt{4^2x^2}}
\\\\\\
\cfrac{3y^2\sqrt{7y}}{4x} [/tex]