The factored expression is: 3(5n-6)
The GCF between 15 and 18 is 3. So you would divide both numbers by 3. That would leave you with 5 taking the place of 15 and 6 taking the place of 18. Then you would fill in the blanks. Three multiplied by the expression 5n-6 would give you your original expression of 15n-18.