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 mixed up the values of the slope, $\displaystyle-\frac{{1}}{{8}}\usepackage{cmbright}\Huge\$ , and the y-intercept, 4, when substituting them into the equation.