Why do we have to canal the part out?

he's just saying that the positive displacement (top triangle) cancels out the negative displacement (bottom triangle) since they are the same
(both have an absolute value of 1/2 x (4-3)s x 4m s -1 = 2 m). It is quite intuitive from the graph. Therefore no need to go and calculate, saving time.

Alternatively, if you want to calculate it, you will get 2m for the top triangle and -2m for the bottom triangle. then your displacement is 9m + 2m - 2m = 9m