Substitute the known pH into the formula, and represent $\displaystyle{H}+\usepackage{cmbright}\Huge\$ with the variable x:

$\displaystyle{6.6}=-{\log{{x}}}\usepackage{cmbright}\Huge\$

Multiply both sides by $\displaystyle-{1}\usepackage{cmbright}\Huge\$ : $\displaystyle-{6.6}={\log{{x}}}\usepackage{cmbright}\Huge\$

Rewrite as an exponential equation to solve for x: $\displaystyle{x}={{10}}^{{-{{6.6}}}}\usepackage{cmbright}\Huge\$