[tex]m_{ST}=-1[/tex]
1) Looking at that diagram, we can see that Point S shares the same x-coordinate from point Q since they are on the same Vertical Line. So, it's safe to say that the x-coordinate of Point S is x=-2
2) And point S is at the same horizontal axis of point P. So for the same principle, we can say that the y-coordinate of point S is y=4
S(-2,4)
3) So let's find the Fourth point of that figure, so let's find the slope of PQ
[tex]\begin{gathered} m_{PQ}=\frac{4-1}{1-(-2)}=\frac{3}{-1+2}=\frac{3}{3}=1 \\ \\ m_{ST}=-1 \end{gathered}[/tex]
Since PQ is perpendicular to ST then the slope is the opposite reciprocal of 1, i.e. -1
