Jack is 2 times as old as lacy. 6 years ago the sum of their ages was 24. how old are they now?

Lucy = x 

now we set up our equation 

if we know that 6 years ago their ages were 18 combined, then we know that something - 6 = 18

 since we also know that jack is 2 times lucy now 

then the entire formula would be

 2x - 6 = 18 to solve we add 6 to both sides 2x - 6 + 6 = 18 + 6 the 6s on the left cancel because when you have a positive and negative of the same number it always = 0 we get 2x = 24
 
then divide both sides by 2 to get the answer
 2x / 2 = 24 / 2 the 2s on the left cancel

and you get x = 12