2 answers:
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If you choose to fully simplify this expression, then the answer is b^3
If you just want to convert to rational exponent form, then the square root will be replaced with an exponent of 1/2. The rule is sqrt(x) = x^(1/2)
So,
sqrt(b^6) = (b^6)^(1/2)
which can be written as
note how the exponents 6 and 1/2 multiply to 3, which is why the fully simplified version is b^3