Question 816364
{{{drawing(300,300,-18,7,-10,15,
grid(0),
triangle(-13,-1,-15,-1,4,9),
locate(-12,-1.5,Q),locate(-16,-1.5,R),locate(4,10.5,S)
)}}}
QR is part of the horizontal line {{{y=-1}}} .
If QS were a vertical line, the x-coordinate of S would be {{{-13}}} ,
just like for Q.
If RS were vertical, the x-coordinate for S would be :{{{-15}}} .
Neither QS nor RS are vertical lines,
so there is no right angle at Q or at R.
Is there a right angle at S?
Without calculating, we see that the length of QR is 2 units,
while QS and RS are much longer, 
So the angle at S is the smallest angle, and cannot be a right angle.
 
NOTES:
Maybe that was not the way your teacher expects you to get to the answer,
but it is the shortest way to the answer.
Maybe there is a typo,
and I just gave you the right answer to the wrong problem.