how do you figure out if its growth or decay?

Generally speaking, a quantity that's growing increases with time, and a quantity that's decaying decreases with time.

I don't understand your question.

Say you buy a car, normally its value will 'decay' or depreciate

Say it costs $20,000 and the price mininshes by 8% every year. 8% = 0.08

then

at the end of the first year it will be worth      20000(1 - 0.08)^1

at the end of the second year it will be worth 20000(1-0.08)^2

at the end of the nth       year it will be worth 20000(1-0.08)^n

OR

if you invest money in the bank you expect it to grow or appreciate in value

Say you invest $20000 and the price appreciates by a set 4% each year  4%=0.04

at the end of the first year it will be worth      20000(1 + 0.04)^1

at the end of the second year it will be worth 20000(1+0.04)^2

at the end of the nth       year it will be worth 20000(1+0.04)^n

Could be cancer or an abcess...or just a cavity.... you should ask your dentist.