This cannot be evaluated because b is negative, so b^3 is negative. The whole expression is therefore negative, and the log of a negative number cannot be evaluated. The positive square root of c is implied. If the negative square root is used, then b=5 (effectively) and the log becomes log(9*125/2sqrt(2))=log(1125/2sqrt(2))=2.6 approx (log to base 10).