Take another look at the original sentence:

Ursula clutched her head, trying to remember the correct formula to solve the statistics problem, the only thing she could focus on, however, was the stupid fight she had in the car with her boyfriend, Mac.

You chose to fix it the correct way:

Ursula clutched her head, trying to remember the correct formula to solve the statistics problem; the only thing she could focus on, however, was the stupid fight she had in the car with her boyfriend, Mac.

Ursula clutched her head ... begins a complete sentence. The only thing she could focus on, however, was the stupid fight ... starts the second complete sentence. You are always correct when you use a semicolon between the two main clauses that create the comma splice.

Go to the next sentence.

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