Try this way: it is possible to make up the system of two equations using the coordinates of points C and D: 31=k*(-19)+b - if to substitute the coordinates of point C; -9=k*1+b - if to substitute the coordinates of point D. [tex] \left \{ {{-19k+b=31} \atop {k+b=-9}} \right. \ =\ \textgreater \ \ \left \{ {{20k=-40} \atop {k+b=-9}} \right. \ =\ \textgreater \ \ \left \{ {{k=-2} \atop {b=-7}} \right. [/tex] y=-2x-7