6x + 15 is the formula for the triangle's perimeter.
Let x represent the triangle's bottom.
The length of one side of a triangle equals 12 times the length of the triangle's bottom, so:
4x + 12 is the side length.
The last side of the triangle is three times longer than the bottom, resulting in:
x + 3 is the length of a side.
Triangle perimeter = x + (4x + 12) + (x + 3)
6x + 15 = Perimeter
6x + 15 is the formula for the triangle's perimeter.