In this question the main information's are already provided. Based on them the answer to the question can be easily deduced.
Let us assume the width of the poster = x
Then the length of the rectangle = 3x
The perimeter of the rectangle = 24 inches
we already know
Perimeter = 2 ( Length + width)
24 = 2( 3x + x)
24 = 2 * 4x
24 = 8x
x = 24/8
= 3
So the width of the rectangle is 3 inches
Length of the rectangle = 3x
= 3 * 3
= 9
So the length of the rectangle is 9 inches.