The expression for the perimeter as well as the choices are not properly shown. Therefore, I cannot give you an exact answer.
However, I can help you with the concept.
In an equilateral triangle:
1- All sides are equal
2- All angles are equal
The perimeter of any triangle can be calculated as follows:
perimeter = first side + second side + third side
Since the three sides of an equilateral triangle are equal and assuming that each has a length of s units, the perimeter would be:
perimeter = s + s + s
perimeter = 3s
Now, if we are given the perimeter and we want to get the side length. all we have to do is divide the perimeter by 3.
Examples:
1- If the perimeter is 60:
side length = 60/3 = 20 units
2- If the perimeter is 6x + 15
side length = (6x+15) / 3 = 2x + 5 units
Hope this helps :)
