What does "i" stand for in the amortization formula?

Which value of c would make he following expression completely factored? 8x+cy

2people are trying different phones to see which one has the better talk time. chatlong has an average of 18 hours with a standard deviation of 2 hours and talk-a-lot has an average of 20 hours with a standard deviation of 3. the two people who tested their phones both got 20 hours of use on their phones. for the z-scores i got 1 for the chatlong company and 0 for talk-a-lot. what is the percentile for each z-score?

Rewrite so the subject can be y in the formula d=m-y/y+n