Simple Question About Pendulum I Can't Do

[tweety_2479]

Simple Question About Pendulum I Can't Do !

The motion of the bob of a pendulum is described by

theta = 3cos(3t) - sqrt3 sin(3t)

where theta is the angle of the bob from the vertical. What is the amplitude of this motion??

as an attempt, as i know what the answer is, but just don't know how to get to it i substituted the answer for theta and tried to find what t was
but it didnt help i don't really no where to start for this probelm

thanks if youcould help

 

[Hootenanny]

HINT: How do you find the maximum of a function?