Let $\displaystyle{t}=\usepackage{cmbright}\Huge\$ the number of T-shirts sold and let $\displaystyle{s}=\usepackage{cmbright}\Huge\$ the number of pairs of shorts sold. Then:

(A) $\displaystyle{t}+{s}={39}\usepackage{cmbright}\Huge\$ (B) $\displaystyle{12}{t}+{9}{s}={414}\usepackage{cmbright}\Huge\$

Solve (A) for t: $\displaystyle{t}={39}-{s}\usepackage{cmbright}\Huge\$

Substitute into (B): $\displaystyle{12}{\left({39}-{s}\right)}+{9}{s}={414}\usepackage{cmbright}\Huge\$

Multiply, and combine like terms: $\displaystyle{468}-{3}{s}={414}\usepackage{cmbright}\Huge\$

The solution of this equation is $\displaystyle{s}={18}\usepackage{cmbright}\Huge\$ , which gives you $\displaystyle{t}={39}-{18}={21}\usepackage{cmbright}\Huge\$ .