Chapter 7 - Section 7.2 - Rational Exponents - Exercise Set - Page 424: 51

$-b$

Using laws of exponents, then, \begin{array}{l} \dfrac{b^{1/2}b^{3/4}}{-b^{1/4}} \\\\= -b^{\frac{1}{2}+\frac{3}{4}-\frac{1}{4}} \\\\= -b^{\frac{2}{4}+\frac{3}{4}-\frac{1}{4}} \\\\= -b^{\frac{4}{4}} \\\\= -b .\end{array}