Multiply both sides by 5 to clear the fractions: $\displaystyle{3}{y}+{35}={15}-{y}\usepackage{cmbright}\Huge\$

Subtract 35 from both sides: $\displaystyle{3}{y}=-{20}-{y}\usepackage{cmbright}\Huge\$

Add $\displaystyle{y}\usepackage{cmbright}\Huge\$ to both sides: $\displaystyle{4}{y}=-{20}\usepackage{cmbright}\Huge\$

Divide both sides by 4: $\displaystyle{y}=-{5}\usepackage{cmbright}\Huge\$