Choose what the expressions below best represent within the context of the word problem. if sarah is 24 years younger than her mother and if the sum of their ages is 68, how old is sarah? x best represents . x - 24 best represents .

Hello Kiddo. 

This question is a bit confusing to me, but I think I grasped the concept. 
1. x best represents Sarah's mom's age. 
2. x - 24 best represents Sarah's mom's age minus how much younger Sarah is than her mom. 
3. The expression would look a little like this:
2x-24 = 68
    + 24  +24
2x = 92 
/2     /2
x = 46.

Which means, Sarah is 22 and her mom is 46 years of age. 
If you need more clarification, I'd be more than happy to elaborate further.