Ineed with this: what expression represents the perimeter of the quadrilateral? the sides are 15+3y, 5y+13, 7-2y, and 2y+4. the answer choices are: 15y+39, 8y+39, 30y+18, and 29y+18. i got 12y+39 instead. !

Just add them all.

(15+3y) + (5y+13) + (7-2y) + (2y+4)

Add common terms:

(3y+5y-2y+2y) + (15+13+7+4)
(8y) + (39)
8y + 39 is your answer.

(You add all sides because when you want to find perimeter, you add up all sides).