It appears that you tried to multiply by $\displaystyle\frac{{h}}{{2}}\usepackage{cmbright}\Huge\$ . In order to clear the fraction, you must multiply both sides by the reciprocal of that fraction, $\displaystyle\frac{{2}}{{h}}\usepackage{cmbright}\Huge\$ . You then tried to isolate b by adding a. You will need to subtract a from both sides to isolate the variable b.