How to find average rate of change

How would I find the average rate of change over 8 minutes, of a person that runs at a rate of $v(t)=x\mathrm{sin}({x}^{2}-7x)$ ft/min? I missed when this was taught and I have no clue on how to do it. Help is greatly appreciated.

How would I find the average rate of change over 8 minutes, of a person that runs at a rate of $v(t)=x\mathrm{sin}({x}^{2}-7x)$ ft/min? I missed when this was taught and I have no clue on how to do it. Help is greatly appreciated.