First, simplify the fraction in the square root:
sqrt(125/126)
Then, use the rulesqrt(a/b)=sqrt(a)/sqrt(b):
sqrt(125)/sqrt(126)
Now, onto simplifying the radicals on the top and bottom: split the square roots using the rule sqrt(ab)=sqrt(a)*sqrt(b):
(sqrt(25)*sqrt(5))/(sqrt(9)*sqrt(14)) (see notes below if this doesn't make sense)
Simplify sqrt(25) and sqrt(9):
(5*sqrt(5))/(3*sqrt(14))
And that's as far as you can go!
Note on simplification of sqrt(125) & sqrt(126):
The way I used to simplify these is to find all the prime factors:
sqrt(125)= sqrt(5) * sqrt(5) * sqrt(5)
sqrt(126) = sqrt(2) * sqrt(3) * sqrt(3) * sqrt(7)
Since sqrt(5) * sqrt(5) = sqrt(5)^2 = 5 and sqrt(3) * sqrt(3) = sqrt(3)^2 = 3, you can simplify these to:
sqrt(125)= 5 * sqrt(5)
sqrt(126) = sqrt(2) * 3 * sqrt(7)
Using the rule sqrt(ab)=sqrt(a) * sqrt(b) on the second expression:
sqrt(125)= 5 * sqrt(5)
sqrt(126) = 3 * sqrt(14)
And you end up in the same place as the first method