Hi I am having a hard time figuring out how to finish off this problem any help would be great thanks.

Otis taylor plots the price per share of a stock that he owns as a function of time and finds that it can be approximated by the function.

S(t) =t(25-5t)+18

Where t is the time (in years) since the stock was purchased. Find the average price of the stock over the first five years.

I have started the problem according to my book and I am stuck here is where I have gotten too in the process.