The vertices of a parallelogram is given as A(1,2),B(4,y),C(x,6),D(3,5).
It is known that,
(x3+x12,y3+y12)=(x4+x22,y4+y22)
So,
(x+12,6+22)=(3+42,5+y2)
(x+12,4)=(72,5+y2)
Comparing both sides,
x+12=72
x+1=7
x=6
And,
5+y2=4
5+y=8
y=3
Therefore,
x=6,y=3