Explanation:
you did not say what needs to be done.
simplify ?
sqrt(12/b²)
12 = 4×3
4 is a square number (2² = 4).
3 is not.
b² is per definition a square "number".
in a multiplication or division we can apply the square root (or any root) individually to the different factors involved.
so,
sqrt(12/b²) = sqrt(12)/sqrt(b²) = sqrt(4×3)/b =
= 2×sqrt(3)/b
FYI - about your question : if somebody does not know (how to get) the answer, he/she will also not be able to doubtlessly determine, if a specific answer is right ...