a. 18 units, 18 units, 13 units
P = 49 means the sum of side lengths is 49. Only answer choice A has numbers that sum to 49.
As here, you often do not need to work a multiple-choice problem. You only need to check to see which answers make sense.
If you want to actually compute the side lengths, you know the two sides with a hash mark are the same length, so the perimeter is ...
P = 49 = (2n+2) + (2n+2) + (2n-3) = 6n +1
48 = 6n . . . . . subtract 1
8 = n . . . . . . . divide by 6
2n +2 = 2·8 +2 = 18 . . . . . . . matches only the first answer choice.