The equation Pequals3s represents the perimeter P of an equilateral triangle with side length s. What is the perimeter of an equilateral triangle with side length 6 mi ?

They gave you the equation. P = 3s.

P = perimeter

s = the side lengths.

The length of the triangle is 6 mi. Plus in your 6 in the s. That equals 18 mi.

You multiply 3 because it is an equilateral triangle - all sides are equal. 3 (6) is the same as 6+6+6