I = PRT, that's the formula. The I means the interest, the P is the principal amount, the R is the rate (percentage) and the T is the time. You would have to solve for the missing letter, which in this case is the I (Interest). So, the formula for this question would be I = 2600 x .7 x 6 . You would multiply all the numbers together, and whatever you get is I. So the answer is 10,920. Hope this helped :) If it's wrong put do 2600 = P x .7 x 6 as the formula.