```Question 135980
Since you have three vertices of the rectangle, you have two sides defined, namely the line segment from (1,5) to (4,5) and the line segment from (1,5) to (1,0).  The segment from (1,5) to (4,5) must be parallel to the segment between (1,0) and your missing point (x,y), and the segment from (1,5) to (1,0) must be parallel to the segment between (4,5) and your missing point (x,y).

Since the y coordinates of (1,5) and (4,5) are equal, the equation for the line containing the line segment connecting these two points must be the equation of a horizontal line, namely {{{y=5}}}.  Since the segment between (1,0) and (x,y) is parallel to this line, it must be a horizontal line as well, and since the y-coordinate of one of the points is 0, all of the y-coordinates must be 0 and the equation must be {{{y=0}}}.

Using similar logic, the equation for the segment from (1,5) to (1,0) must be {{{x=1}}} and therefore the equation for the segment from (4,5) to (x,y) must be {{{x=4}}}.

Hence your point is the intersection of the two lines represented by {{{y=0}}} and {{{x=4}}}, so the ordered pair is the solution set of the system:

{{{y=0}}}
{{{x=4}}}

Which is (4,0)```