A parallelogram is a quadrilateral that has two pairs of parallel sides.
There are several rules involving the angles, the sides or the diagonals of a parallelogram.
Angles of a parallelogram
 Opposite angles of a parallelogram are congruent.
 Consecutive angles of a parallelogram are suppementary.
Sides of a parallelogram
 The opposite sides of a parallelogram are congruent.
Diagonals of a parallelogram
 The diagonals of a parallelogram bisect each other.
Find x and y 

Consecutive angles are supplentary:
(6x+10)+(2x+10)=180
8x+20=180
8x=160
x=20º
Opposite angles are congruent:
2(20)+10=y
y=50º
Find the perimeter P 

Opposite sides are congruent:
20=x+3
x=17
P=20+2(17)+17+3+2(17)=108 units.