Okay, we start with the formula for simple interest, A = P(1 + rt). However, we are really interested in the p*rt part of the formula. Why? r is the interest rate, p is the principal and t is the time.
Let's solve for r using the first loan info.
We have r*6*15000=2610 ------> r=2.9%. From our calculation the interest is 2.9% per year.
For the $32,000 loan in 8 yrs, the interest amount is 8*2.9%*32000=$7424