Monomials are divided by dividing the coefficients and dividing the variables. When you have the same variable, you can subtract the exponents. A variable with a negative exponent can be rewritten with a positive exponent in the denominator.

$\displaystyle\frac{{{81}{a}{b}}}{{{9}{{a}}^{{4}}{b}{{c}}^{{4}}}}={9}{{a}}^{{{1}-{4}}}{{b}}^{{{1}-{1}}}\cdot\frac{{1}}{{{{c}}^{{4}}}}={9}{{a}}^{{-{3}}}{{b}}^{{0}}\cdot\frac{{1}}{{{{c}}^{{4}}}}=\frac{{9}}{{1}}\cdot\frac{{1}}{{{a}}^{{3}}}\cdot{1}\cdot\frac{{1}}{{{c}}^{{4}}}=\frac{{9}}{{{{a}}^{{3}}{{c}}^{{4}}}}\usepackage{cmbright}\Huge\$