The equation is the slope form of the line with slope -1 going through the point (1, -2), so you know that the line goes through (1, -2). However, when $\displaystyle{\left({x},{y}\right)}={\left(-{2},{1}\right)},{y}+{2}={1}+{2}={3}\usepackage{cmbright}\Huge\$ and $\displaystyle-{\left({x}-{1}\right)}=-{\left(-{2}-{1}\right)}={3},\usepackage{cmbright}\Huge\$ so (-2, 1) also lies on the line.