You need to tell us what you know. Provide some concrete numbers.
By "period", do you mean the number of periods, or do you mean the frequency?
If you mean number of periods, you might be able to use NPER to calculate the number directly without the use of Goal Seek.
But that depends on what you know, specifically: periodic rate, periodic payment, initial loan amount, and final balance (usually zero). You also need to know if payments are in arrears (end of period, which is typical) or in advance (beginning of period, which is typical of leases).
PS.... But of course, if you goal (no pun intended) is simply to learn how to use Goal Seek, even though NPER might do the job, you can simply use NPER as a double-check.
