I am not a computer science major, but I am a philosopher. We invented symbolic logic, and we all have to learn it.
What you have looks mostly correct. “Unless” can be translated as “if not,” but you can also use disjunction (aka “or”) as you have. So “Axel will fail the maths unit unless Maria helps him with his maths homework” could be translated either as Q v R or ~Q → R. Notice that the argument has a conditional conclusion, so you don’t need to end up with an atomic sentence at the end of the problem. Therefore, your symbolic representation of the argument should be:
~P → Q
Q ˅ R
————
P → R
Alternatively, it could be:
~P → Q
~Q → R
————-
P → R
Since you don’t know whether or not Maria will go out, you cannot reach any conclusion about whether or not Axel will fail the maths unit. (So not only don’t you need to end up with an atomic sentence at the end of the problem, you cannot do so with the information provided.)