Translation:If he strived to learn, he would be a mathematics professor.
could be rather translates to olabilir because it indicates possibility.
On the other hand it's also a fact that we occasionally use olur instead olabilir. So their meanings are very close and they can be replaced with each other in this sentence. But still you should prefer the ending -AbIl when translating can/could.
In my neck of the woods, "If he had worked harder to learn, he would be a professor of mathematics." Yes, this is one of those times when a little adjustment would be helpful. It appears that there are many interpretations.
Oddly, while studying this, I realized--I think--how different "striven" is from "struggled". "Striven" appears to mean that he had made an effort to learn to his utmost. "Struggled", on the other hand, means--in my mind--that, even with his best effort, he might not have achieved his goal.
Is it for the past tense or present? Because I wrote: "If he strives to learn, He will be a mathematics professor". It accepted my answer and said: (Another correct solution: If he strived to learn, he would be a mathematics professor.) Wow, and How?
In addition, the hint for (için) gives (in order to) however it's not accepted here on Duolingo, I tested that.