Explanation

given: y = 2 sin (400$\pi$ t - 0.3x)

we recall the general equation of a progressive wave

y = Asin($\omega$ t - $\phi$ x)

$\omega$ = 2$\pi$ ft - $\frac{2πx}{\lambda}$

by comparison

400$\pi$ = 2$\pi$ f

f = $\frac {400}{2}$ = 200Hz

given: y = 2 sin (400$\pi$ t - 0.3x)

we recall the general equation of a progressive wave

y = Asin($\omega$ t - $\phi$ x)

$\omega$ = 2$\pi$ ft - $\frac{2πx}{\lambda}$

by comparison

400$\pi$ = 2$\pi$ f

f = $\frac {400}{2}$ = 200Hz