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 ignored the negative sign of the slope. The slope of the line is $\displaystyle-{3}\usepackage{cmbright}\Huge\$ , not 3.