John,
Simple answer: Catholics believe that scripture is materially sufficient for salvation but not formally sufficient
Here is a very good article that explains the difference.
The Church from the beginning turned to Tradition to interpret the Written Word of God. The Church was answering your question nearly 1,600 years ago and 42,000 protestant denominations each believing differently using the same bible is proof that scripture is materially, not formally sufficient. There needs to be a an authoritative and infallible teacher…and there is.
**“Here perhaps, someone may ask: Since the canon of the Scripture is complete and more than sufficient in itself, why is it necessary to add to it the authority of ecclesiastical interpretation? **As a matter of fact, we must answer Holy Scripture, because of its depth, is not universally accepted in one and the same sense. The same text is interpreted different by different people, so that one may almost gain the impression that it can yield as many different meanings as there are men. Novatian, for example, expounds a passage in one way; Sabellius, in another; Donatus, in another. Arius, and Eunomius, and Macedonius read it differently; so do Photinus, Apollinaris, and Priscillian; in another way, Jovian, Pelagius, and Caelestius; finally still another way, Nestorius.
Thus, because of the great distortions caused by various errors, it is, indeed, necessary that the trend of the interpretation of the prophetic and apostolic writings be directed in accordance with the rule of the ecclesiastical and Catholic meaning.” – Commonitory for the Antiquity and Universality of the Catholic Faith 2 (A.D. 434).
John, it’s also hard to argue for sola scriptura when one doesn’t even have all the books in the bible…how were 7 books removed from your bible and by who’s authority? Even the Original KJV 1511 had 73 books…