It would be 0 because as you go along it doesn't go up but keeps moving. An undefined slope is up and down/vertical.
So to find the mean you add them all together and divide by the number of numbers there is so 11+15+19+17+23+20+17+18+21=174 and there are 10 numbers so it would be 174/10 which is 17.4 so you answer is 17.4
