Clara is building a triangular garden. she wants the length of the longest side to be three more than twice as long as the length of the shortest side, and the third side will be twelve feet long. what expression could she write to determine the perimeter of the triangle if s represents the length of the shortest side? (recall that perimeter is the distance around a shape.)

Alrighty-
So here we go.
Let's first go over any given values.

Shortest side = s

And we know that
3rd side = 12

Now for the longest side
"Three more" means +3
"...twice as long as the shortest side"
Means 2s

Longest side = 3+2s

Now we put this in the correct format:

P = 12 + s + (3+2s)

We can simplify further:
P = 12 + s + 3 + 2s
P = 15 + s + 2s
P = 15 + 3s

ANSWER: P = 12 + s + (3+2s)

SIMPLIFIED: P = 15 + 3s