Explanation:
Question Says:-
An equilateral triangle is a triangle with three equal sides. The perimeter of an equilateral triangle varies directly with the length of one side. What is an equation that relates the perimeter p and length of a side?
Answer:-
See, let us consider the sides as l as per question.
And, perimeter as p as per question.
We know that:-
The sum of all the boundary gives us the perimeter of the figure.
It is Equilateral Triangle, so the equation will be:-

Now, adding it up, we get

So, 3l=p relates the side and perimeter.
Hope it helps :)