Do not forget to subtract 1 from the first number when you regroup. You need to subtract $\displaystyle{2}\frac{{15}}{{12}}-{2}\frac{{8}}{{12}}\usepackage{cmbright}\Huge\$ , not $\displaystyle{3}\frac{{15}}{{12}}-{2}\frac{{8}}{{12}}\usepackage{cmbright}\Huge\$ .