Exponential
The first year, he would earn 8% interest on the $400 deposited, so he'd have $432. The next year, he would earn 8% interest on the interest earned the previous year in addition to the amount deposited, so he'd have $466.56.
His bank balance would follow the compound interest equation which is exponential.
= 400 × 1.08^t
where t is the number of years since the deposit
