Subtract $\displaystyle{3}{b}\usepackage{cmbright}\Huge\$ from both sides: $\displaystyle{p}-{3}{b}={10}{a}\usepackage{cmbright}\Huge\$

Divide both sides by 10: $\displaystyle\frac{{{p}-{3}{b}}}{{10}}=\frac{{{10}{a}}}{{10}}\usepackage{cmbright}\Huge\$

Simplify: $\displaystyle\frac{{{p}-{3}{b}}}{{10}}={a}\usepackage{cmbright}\Huge\$