Find the GCM (which is 3) for all of the numbers
Once you do that divide all the numbers by the GCM so it would be 27/3 = 9 then -9x/3 = -3x, then +15y/3 = 5y
Once you do that, you have to put the GCM in front of the expression so it would be 3(9 -3x+5y)
then you are done :)