To find a missing factor, you must find a number that can be added to 7 for a sum of 11, and multiplied to 7 for a product of 28.
Find all the multiples of 28.
Multiples of 28 = {1,2,4,7,14,28}
7 is a multiple of 28, as it multiplies with 4 for a product of 28. 7 and 4 added together create a sum of 11.
The missing width is x+4.