The goal is simple, the question was worded by an idiot who does not under stand English.

They want you to make one of the groups into a ten. Then add the two new groups to get the answer.

9+1=10; or 8+2=10.

So you would take 2 from the box of 9 and put it in with the box of 8, making a full box of 10, leaving you with 7. 10+7=17.

Algebra light.

A lot of us find 5's and 10's to be easier to add/subtract. by making it as close to 10 as possible makes the adding easier.