You may have chosen to multiply the fractions before simplifying each, using the product rule, and correctly getting $\displaystyle\frac{{{x}}^{{20}}}{{{x}}^{{5}}}\usepackage{cmbright}\Huge\$ . You may have divided the exponents at this point, but you should have subtracted. The correct answer is $\displaystyle{{x}}^{{15}}\usepackage{cmbright}\Huge\$ .