Verified Answer :

Simplifying
14x + -21xy
 
Factor out the Greatest Common Factor (GCF), '7x'.
7x(2 + -3y)

Final result:
7x(2 + -3y)
 
that is the answer on how to do it

You need to find the common number/letter in order to factorise and it should be like this:
x(14-21y)
Hope it helped...