Answer:
or
Step-by-step explanation:
Given the following expression:

You can follow these steps in order to simplify it:
1. You need to apply Distributive property:

2. And now you must add the like terms. Then:

Therefore you get the following answer:
or
The perimeter of an equilateral triangle is just 3s as all the sides are, well, equal :) so
P=3s and we are told that P=24.6 so
3s=24.6
s=8.2 cm