Your are looking for an average over a time span.The derivative would give you a velocity at one point at time. the 40,44.1 and 90 they got putting the 3 times they are interested in namely 2,2.1 and 3 seconds that mark the end of intervals they are looking at X=10T^2 for 2 is 40,for 2.21 is 4.41 and for 3 is 90.