Question

2. Forrest Lumber uses the function
S(t) = -105t + 840

to determine the salvage value S(t), in dollars, of a table saw t years after its purchase. How long will it take the saw to depreciate completely?
9 years
10 years
8 years
7 years

Answer 1

you could simply use trial and error as you only have four options,
for example replace t with 9 and see if it equals 0
if the number is below 0 you need less time, if above try with a higher number