The formula for the perimeter of a triangle is
P = (side number 1) + (side number 2) + (side number 3)
An equilateral triangle has all three sides equal.
So if one side is 2x, all three sides are 2x.
P = 2x + 2x + 2x = 6x. That's the perimeter in terms of x.
Answer: 6x
Edwin