Think of all the factors that equal 28 and line them up smallest to greatest.
1, 2, 4, 7, 14, 28
Look at all the numbers, and try adding some of them together.
Let's start with 1.
1+2=3 ×
1+4=5 ×
1+7=8 ×
Well, 1 isn't one of the factors because no matter which factor you add it to, it'll be either larger or smaller than 9.
Let's try 2.
2+1=3 ×
2+4=6 ×
2+7=9 √
The factors 2 & 7 added together equal 9.
Hope this helps.
