For each number, generate a factor tree. This is where you break a number down into smaller factors in a visual "tree" like style. The idea is to factor down to the prime factors and then you circle the common primes between each number. The values you circle are then multiplied if you circle more than one value in each tree.
Xiao factored correctly but didn't pull out the GCF. So he didn't factor fully. In the case of 60 and 90, the GCF is 30. This is the largest shared factor between the two values. So we can pull out 30 to get 60+90 = 30*2+30*3 = 30*(2+3)
I'm basically using the distributive property in reverse. You can distribute the 30 back in and get 60+90 again. Notice how the inner stuff 2 and 3 have no factors in common other than 1.
