"Perimeter" is the distance all the way around the drawing. If the drawing has sides, then the perimeter is the SUM of the lengths of all the sides. It's exactly the distance an ant would have to walk along the line, to get all the way around to where he started from.
To get the perimeter of the triangle, you have to add up the lengths of the three sides.
(2a - 3) + (2a) + (3a + 1) =
2a - 3 + 2a + 3a + 1 .
Add up all the 'a's: 2a + 2a + 3a = 7a
Add up all the just plain numbers: -3 + 1 = -2
Write them together, as a binomial: 7a - 2
THAT's your expression for the perimeter.
