I have

$\displaystyle

y=5 sin (4t) + 9 Cos (4t)

$

i am told to put it in the form; $\displaystyle R Sin (4t + \alpha)$where $\displaystyle 0 < \alpha < (\pi/2)$

angular freq, w= 4 rads,

and the period is $\displaystyle t= 2 \pi/w$ = 1.57 secs

how to you find the amplitude???