The length of the larger rectangle is x+4, and the length of the smaller one is x.
The proportions of the widths and the lengths have to be the same; thus, we can create an equation.

On the left side, multiply both the numerator and denominator by 2. Now, we have

Set the numerators equal to the numerator and denominator equal to the denominator.
x+4=10
x=6
Both of these equations are true when x=6, which is our answer. Hope this helps! :)