Factor out first the radicand.
sqrt (-48) = sqrt ( -16 x 3)
= sqrt ( -1 x 16 x 3)
Calculate for the square roots of the factors.
= sqrt (-1) x 4 x sqrt (3)
The square root of -1 may also be written as i which means that the value is imaginary. Thus, the final answer would be,
= 4i (sqrt 3)