The position of an object moving along a line is given by p(t) = 4t - sin(( pi )/3t) . What is the speed of the object at t = 9 ?

speed $\frac{\mathrm{dp} \left(t\right)}{\mathrm{dt}} = 4 + \frac{\pi}{3}$
Differentiate $p \left(t\right)$ with respect to time $t$ then substitute $t = 9$
$p ' \left(t\right) = \frac{d}{\mathrm{dt}} \left(4 t\right) - \frac{d}{\mathrm{dt}} \left(\sin \left(\frac{\pi t}{3}\right)\right)$ then substitute $t = 9$