The slope of the line containing (2,7) and (5,y) is $\displaystyle\frac{{{y}-{7}}}{{{5}-{2}}}=\frac{{{y}-{7}}}{{3}}\usepackage{cmbright}\Huge\$ , which will equal -3 only if $\displaystyle{y}-{7}=-{9}\usepackage{cmbright}\Huge\$ , so y = -2.