Subtract 45 from both sides:

$\displaystyle-{6}{a}+{45}-{45}\le{3}-{45}\usepackage{cmbright}\Huge\$ or $\displaystyle-{6}{a}\le-{42}\usepackage{cmbright}\Huge\$

Divide both sides by $\displaystyle-{6}\usepackage{cmbright}\Huge\$ :

$\displaystyle\frac{{-{6}{a}}}{{-{{6}}}}\ge\frac{{-{42}}}{{-{{6}}}}\usepackage{cmbright}\Huge\$ or $\displaystyle{a}\ge{7}\usepackage{cmbright}\Huge\$