Beth plays a video game in which she starts with 0 points. in round 1 she loses 3 1/2 points; in round 2 she wins 28 1/2 points; and in round 3 she loses another 3 1/2 points. what is her final score

Her final score should be 21. You can add both 3 1/2 and 3 1/2 which will give you 7. Then substract it from 28. 

It would be like first round she started with 0 and she loses means points goes negetive
-3 1/2  it is a mixed number solve it==7/2
round 2 she wins 28 1/2    mixed number solved to 57/2
at round  2 her score will be -7/2+57/2=50/2=25

at round 3 she loses again 7/2 means 25-7/2=43/2 will be her score

The answer is 6 weeks