Seriously, I suggest you ask your teacher for a sit-down together, to review some of the previous work. Math builds like brick wall. When there's a level that you don't understand, then you can't build more levels on top of it. You can't keep going through math getting answers from other people. Sooner or later, you need to understand how to solve the questions on your own, and the longer you put it off, the tougher it could get.
In these questions, "+" is always 3 more than 'a' is.
20). "+" is 3. That's 3 more than 'a'. So 'a' is zero.
21). 'a' is 1. "+" is 3 more than 'a'. So "+" is 4.
22). "+" is 5. That's 3 more than 'a'. So 'a' is 2.
23). 'a' is 3. "+" is 3 more than 'a', so "+" is 6.
