The velocity of an object is given as a function of time by $v(t)=4t-9t^2,$ where $v$ is in $\frac{m}{s}$ and $t$ is in seconds. What is the average velocity of the object over the interval; $t=0$ to $t=3s$ :

$($a$)-21\frac{m}{s}$

$($b$)-23\frac{m}{s}$

$($c$)-63\frac{m}{s}$

$($d$)$ Cannot be calculated unless the positions are given