Calculus problem with average angular velocity

[achiu17]

A wheel has angular position theta = 0 (radians) at t = 0 (seconds). It then rotates with an angular velocity equal to .72t radians/sec. In radians per second, what is the average angular velocity from theta = 4 radians to theta = 9 radians?

I got 3 for my answer but I'm not sure if it's right

 

[haruspex]

Please post your working.