first find the square root of the first term: sqrt(9x^4) = 3x^2

then find the square root of the second terms: sqrt(25y^2) = 5y

*when finding the square roots of variables raised to exponents, divide the exponents by 2.

The answer in a difference of two squares problem is always (sqrt of 1st term + sqrt of 2nd term)(sqrt of 1st term - sqrt of second term)

so your answer is:

(3x^2+5y)(3x^2-5y)