Keywords: 
Expression, variable, factor, GCF, factor 
For this case we have an expression with a variable, in which we must factor the expression using the greatest common factor. By definition we have that the GFC of two numbers is the largest number that is a factor of both numbers. 
Having said that we have the following expression: 

So the greatest common factor between 15 and 65 is 5. So: 

Answer: 
The greatest common factor of the expression 
 is 5, then:
 is 5, then: 
