The slope-intercept form of a line is $\displaystyle{y}={m}{x}+{b}\usepackage{cmbright}\Huge\$ , where m is equal to the slope of the line and b is equal to the y-intercept. You correctly substituted the value for the slope. However, you used the y-intercept as the coefficient of y, instead of replacing b with the value $\displaystyle-{3}\usepackage{cmbright}\Huge\$ .