Financial calculation question: variable interest rate

[Dave Britten]

Suppose I open a home equity line of credit that has a 1.99% introductory rate for 6 months. Then I immediately finance something for, say $4000. After 6 months, the interest rate increases to 2.99%. How would I go about calculating 12 equal payments to have the financed amount fully paid off at the end of 12 months?

Note I'm assuming such a credit line has beginning-period payments like a credit card, and the interest will compound at 1.99% APR six times before changing to 2.99%.