The poynting vector of an electromagnetic wave in vacuum is \(\vec S=\left \{ \left ( 180 W/m^{2}\right )sin^{2} \left [ \left ( 9 rad/m \right ) z+\left ( 3.14\times 10^{8} rad/s\right )t\right ]\right \}\hat{k}\)  what is the frequency of the wave.