One form of the distributive property says $\displaystyle{a}{\left({b}-{c}\right)}={a}{b}-{a}{c}\usepackage{cmbright}\Huge\$ . In particular, $\displaystyle{3}{\left({x}-{1}\right)}={3}{\left({x}\right)}-{3}{\left({1}\right)}={3}{x}-{3}\usepackage{cmbright}\Huge\$ . So the left side of the original equation has been rewritten using the distributive property.