Though you found the correct slope of the line, you made a mistake when finding b. After substituting the point $\displaystyle{\left({7},{6}\right)}\usepackage{cmbright}\Huge\$ for x and y in $\displaystyle{y}=\frac{{1}}{{2}}{x}+{b}\usepackage{cmbright}\Huge\$ , you incorrectly added $\displaystyle\frac{{7}}{{2}}\usepackage{cmbright}\Huge\$ instead of subtracting $\displaystyle\frac{{7}}{{2}}\usepackage{cmbright}\Huge\$ from both sides of the equation.