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PsychrolutesMarcidus
x is the variable for the length of the sides.

Because a equalilateral ∆ has 3 equal sides, they can be represented by one term (x)
BackPacker99
The perimeter of an object is equal to its sides added up.  Because this is a triangle, it will have 3 equal sides.  Since the triangle has 3 equal sides, its perimeter will be equal to one of its side lengths * 3.  

Because the perimeter will be equal to one of its side lengths * 3, x is equal to one of the triangle's side lengths.