a$)$ A current waveform in a robotic arm may be described by$...$

$i_s=13cos(2ft-\frac{}{4}),[A]$

where frequency, $f=1Hz$ and $t$ represents time.

Make time $($ t $)$ the subject of this formula and hence determine a point in time when the current waveform has a magnitude of $+10$ A $.$

I have the answer $,$ but i am struggling to use a graph to plot the current signal