Revenue of Sand Blasting Bolts

Problem: 

The annual revenue for sand blasting bolts over the last ten years can be approximated by $R(t)=0.91t^4-37.5t^3+560t^2-2785t+8156$ where $t=0$ is ten years ago and $t=10$ is today. At what rate was the revenue increasing three years ago?

Answer: 

$\$791.02$ per year