An equilateral triangle sides are all equal to each other. This means that is one side of an equilateral triangle is x + 3 then the sides of the other two triangles as also each x + 3 The formula for perimeter of a triangle is:Perimeter = side 1 + side 2 + side 3Or in the case of an equilateral triangle it would be:Perimeter = 3(side)This means that for the formula of this equilateral triangle you have:Perimeter = 3(x + 3)If asked the three out side of the parentheses can be distributed, meaning that you multiply the outside number (in this case that is 3) to all the number inside the parentheses (in this case that is x and 3)Perimeter = (3 * x) + (3*3)Perimeter = 3x + 9Both of these are correct:P = 3(x + 3)P = 3x + 9Hope this helped!~Just a girl in love with Shawn Mendes