A conjugate (in the context of math) is, simply put, two numbersseparatedby a sign, in which the sign changes.
Example:
The conjugate of a+b is a-b. Thepositivebecame negative.
For your problem (your sign is missing, so I listed two possibilities):
The conjugate of 11+17i is 11-17i
The conjugate of 11-17i is 11+17i