A sum of $20,000 is used to buy a deferred perpetuity-due that pays $2,100 every year for the first 5 years and $1,000 per year thereafter. If the annual effective rate is 6%, find the deferred period.

Note: Please use the Financial Mathematics formulas and don't use excel for this question. I'd be appreciated if it is not calculated by each year. This question is in the book "Financial Mathematics for Actuaries" so I need answer according to it. I'm currently studying for e-xam FM so any related formula would be appreciated. Thanks a lot.