If you did this by applying it to the left addition first, you would get

$\displaystyle{\left(\frac{{2}}{{7}}+\frac{{1}}{{3}}\right)}+\frac{{3}}{{4}}\usepackage{cmbright}\Huge\$ , then applying it to the other addition gives $\displaystyle\frac{{3}}{{4}}+{\left(\frac{{2}}{{7}}+\frac{{1}}{{3}}\right)}\usepackage{cmbright}\Huge\$ .

If you choose to apply the commutative property to the right addition first, you would get $\displaystyle\frac{{3}}{{4}}+{\left(\frac{{1}}{{3}}+\frac{{2}}{{7}}\right)}\usepackage{cmbright}\Huge\$ , then applying it to the other addition gives $\displaystyle\frac{{3}}{{4}}+{\left(\frac{{2}}{{7}}+\frac{{1}}{{3}}\right)}\usepackage{cmbright}\Huge\$ . The results are the same and what you want.