Question 34566
Set up your equation first.  Area = L W
                              28  = (x+3)(x)  (since you said L=x+3, and w=x)

Now you have 28 = {{{x^2 + 3x}}}

This means you have a quadratic to solve, so set it equal to zero, by putting all terms on one side to get

{{{x^2 + 3x -28 = 0}}}

Factor this to get (x+7)(x-4) = 0

Now set both to zero to solve for x  x + 7 = 0   x=-7
                                     x - 4 = 0   x=4

Since you can't have a negative value for length, the answer for x can only be 4

Therefore your width is 4 and Length = 4 + 3 or 7   

x=4