Read it as a string, then - assuming it won't always have a "+" in the middle - process it into a collection of strings: a sequence of numerics becomes a string, a sequence of non-numerics becomes another.
In this case you will end up with three strings:
You can then convert the numerics to numbers, and work out the expression using a
It's not difficult if you do it stage by stage - but this is your homework, not mine, so I'll give you no code!
Sit down and think about it for a few minutes - it's really not as complicated as you seem to think.