k so I have a word problem:

Kathryn bought a doll that decreases in value by $0.60 per year. If she bought it for $15, what will the value be 7 years after she buys it?

I'm THINKING that the formula I use is: Sn = n/2 [ 2a + (n-1)d ]

Is that correct?

a being the original value ( 15 )

d being the difference between ( 0.60 )

n being the number of years ( 7 )

Help? I got something ridiculous like 92.6.