
David W. answered 05/27/19
Music Theory Professor and Composer with 5+ Years Teaching Experience
There's some confusion in your description, so I'm not entirely sure whether you're on the right track or not.
•You don't tonicize a secondary dominant, you use a secondary dominant to tonicize a chord other than the tonic chord.
•Once you've chosen the chord (for instance, F major, the III chord in D minor), you build the dominant 7th chord on the note a perfect 4th below or perfect 5th above—in this case, C. So the secondary dominant you're building here is a C dominant 7 chord, C-E-G-B flat. The Roman numeral would be V7/III.
•Does that help? Please review the language in the assignment again. If they're really asking you to "tonicize a secondary dominant" I'll need some more information to help you.