answer
[tex]y = 3000 * 0.85^x[/tex]
explanation
exponential functions can be expressed in the form [tex]y=ab^x[/tex] where a is the starting amount and b is the ratio
a = 3000
since the value decreases at a rate of 15% = 0.15, the value it retains each year is b = 1 - 0.15 = 0.85
[tex]y = 3000 * 0.85^x[/tex]
