Compound discount question

Edit: I have figured out how to solve this problem

"An account with initial amount $B earns compound interest at an annual effective interest rate i. The interest in the third year is $426, and the discount in the seventh year is $812. Find i."

I know how to solve this problem with a graphing calculator or program that can solve variables, but I'm only allowed to use a regular calculator on the upcoming test. How can I set up the equations in this problem to solve for i in a minimal amount of time?