The resale value of a textbook decreases by 25% with each previous owner. a new textbook is sold for $85. which function represents the resale value of the textbook after x owners?

Since the value decreases by 25% each time, the next cost is the current cost times 0.75 (since new cost is 75% of current cost).
To find the value of the book after x times sold, you can use:
 
Just plug in the number of times it is resold to find the value.

A exponential function would represent it nicely f(x)=b*a^x
Reason being a decrease of %
a = 1+r
r = -25% = -0.25
And b being what the original cost of the book
f(x)=85*0.75^x
Where f(x) is the price after x amount of previous owners