I need some help calculating the velocity?

Question

I need some help on the third part of this question.

A Honda Civic travels in a straight line along a road. Its distance x from a stop sign is given as a function of time t by the equation x(t)= alpha t^{2}- beta t^{3}, where alpha = 1.56 m/s^2 and beta = 4.80×10−2 m/s^3.

Calculate the average velocity of the car for the time interval t=0 to t_1 = 2.00 s
Answer = 2.93 m/s

Calculate the average velocity of the car for the time interval t=0 to t_2 = 3.99 s.
Answer = 5.46 m/s

Calculate the average velocity of the car for the time interval t_1 = 2.00 s to t_2 = 3.99 s.
This is the part I need help figuring out.....

Answer 1

If you trust the first two answers, you can calculate the third as follows:
We want VAve2, where
Total distance = distance1 + distance2
VAveTotal*(t1 + t2) = (VAve1*t1 + VAve2*t2)
VAveTotal = 5.46
VAve1 = 2.93
t1 = 2.00
t2 = 1.99
5.46*3.99 = 2.93*2.00 + VAve2*1.99
VAve2 = (5.46*3.99 - 1.93*2.00) / 1.99 = 9.01 m/s
You can also do this with calculus, but why bother?