Question 53742
Since your biggest problem seems to be converting the problem into an equation, I'll give that a bit more emphasis:

The perimeter of a rectangle is 24 inches. Find the dimensions if its length is 3 inches greater than its width.

Let's define what variables we'll need and what they stand for:

width = x (I named it x because I wanted to; length would work just as fine)
length = x+3 (The length is 3 inches greater than the width)

The perimeter of a rectangle is the sum of the length of all of its sides (width+width+height+height), therefore the perimeter of our rectangle is {{{x+x+x+3+x+3}}}. Since we know that equals 24, we have an equation!

{{{x+x+x+3+x+3 = 24}}}
{{{4x + 6 = 24}}}
{{{4x = 18}}}
{{{x = 18/4}}} or 4 1/2 inches. The length, therefore, is that number plus 3, or 7 1/2 inches.

Checking:

7 1/2 * 2 + 4 1/2 * 2 =
15 + 9 =
24