Question 1070930
The diagonals of a parallelogram bisect each other.
that means that the midpoint of AC is also the midpoint of BD.
Thee coordinates of the midpoint of a segment
are the averages of the corresponding coordinates
of the endpoints of the segment.
The midpoint of AC is ((2+(-1))/2,(0+1)/2) = (1/2,1/2) .
The midpoint of BD is ((p+3)/2,(-2+r)/2) .
So {{{(p+3)/2=1/2}}} <--> {{{p+3=1}}} <--> {{{p=1-3}}} <--> {{{highlight(p=-2)}}}  ,
and
{{{(-2+r)/2=1/2}}} <--> {{{-2+r=1}}} <--> {{{r=1+2}}} <--> {{{highlight(r=3)}}}  .
{{{drawing(300,300,-4,4,-4,4,
grid(1),red(line(2,0,-1,1)),
red(line(-2,-2,3,3)),
line(2,0,-2,-2),line(-1,1,-2,-2),
line(-1,1,3,3),line(2,0,3,3)
)}}}