Question

Three vertices of a rectangle are (-1, 1), (5, 1) and (5, 3). Plot these points on a graph paper and find the coordinates of the 4th vertex.

Answer 1

if 
1)  (-1,1)
2)  (5,1)
3)  (5,3)   
then the 4th coordinate will be (-1,-3)

Answer 2

Your 4 points of the rectangle are

A(−4,5),B(−4,2),C(3,2),D(x,y)A(−4,5),B(−4,2),C(3,2),D(x,y)

From inspection, we can see that AA and BB have the same xx value, and it is most likely that this rectangle is not slanted as BB and CC have the same y value. Let's continue from here:
From point BB to point AA, the yy value increased by 3, so we can say that from point CC to point DD the yy value should also increase by 3 units. We can therefore deduce that the yy value of D=(2+3)=5D=(2+3)=5. Which also is the yyvalue of point AA. Now for the xx values...
From point BB to point DD the x value increases by 7, so we can say that the xxvalue should also increase by 7 from point AA to point DD... Therefore the x value of D is (−4+7)=3(−4+7)=3 (Which is the same xx value as point CC