Respuesta :

Formula version 1:    2*width + w*length = 2x + 2(x+6)

Formula version 2:   2*( width + length) = 2 [ x + (x+6) ]

"Brute force" perimeter / add up all the sides :   X + X + (x+6) + (x+6)

The perimeter is 4x + 12 = 4(x+3)