let the width be x.
length = 3*width = 3*x = 3x.
Perimeter, P = 2*(l + w), P = 30 inches.
P = 2*(l + w)
30 = 2*(3x + x)
30 = 2*(4x)
30 = 8x
8x = 30
x = 30/8 = 15/4 = 3.75
Dimensions, w =x = 3.75 inches, length = 3x =3*3.75 = 11.25 inches.
So dimensions of the rectangle is 11.25 inches by 3.75 inches.
Hope this helps.