Depreciation/Rate of Change (Calculus I)

Problem: The value of a piece of machinery depreciates exponentially. When purchased new, it was worth $70,000. After 4 years, it was worth $55,200. Find the rate of change of the value of the equipment after 4 years.

I started with:

4r = ln (55,200/70,000)

so..

4r = -0.2375322...

divide by 4 on both sides..

r = -.05938...

multiply by initial value (70000)

I got:

$4,156.60

which was not an answer choice, lol

I'm obviously setting this up wrong, any help is appreciated :D