Let $\displaystyle{a}=\usepackage{cmbright}\Huge\$ the number of apple pies sold and let $\displaystyle{b}=\usepackage{cmbright}\Huge\$ the number of blueberry pies sold. Then:

(A) $\displaystyle{a}+{b}={39}\usepackage{cmbright}\Huge\$ (B) $\displaystyle{10}{a}+{14}{b}={458}\usepackage{cmbright}\Huge\$

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

Substitute into (B): $\displaystyle{10}{\left({39}-{b}\right)}+{14}{b}={458}\usepackage{cmbright}\Huge\$

Multiply, and combine like terms: $\displaystyle{390}+{4}{b}={458}\usepackage{cmbright}\Huge\$

The solution of this equation is $\displaystyle{b}={17}\usepackage{cmbright}\Huge\$ , which gives you $\displaystyle{a}={39}-{17}={22}\usepackage{cmbright}\Huge\$ .