Use the distributive property on both sides of the equation to simplify:

$\displaystyle{5}{x}-{15}+{x}={6}{x}-{15}\usepackage{cmbright}\Huge\$

Add like terms on the left side: $\displaystyle{6}{x}-{15}={6}{x}-{15}\usepackage{cmbright}\Huge\$

Subtract $\displaystyle{6}{x}\usepackage{cmbright}\Huge\$ from both sides: $\displaystyle-{15}=-{15}\usepackage{cmbright}\Huge\$

This is true no matter what x is, so the solution is all real numbers.