The speed of a progressive plane wave can be calculated as the product of its wavelength (λ) and its frequency (f). In this case, the frequency is given as 50 Hz, so we just need to find the wavelength.
The equation y = 3 sin 2π(50t - 0.4x) can be rewritten as y = 3 sin 2π(λ(50t - 0.4x), where λ is the wavelength.
From this, we can see that λ = 1/50 x 0.4 = 0.008 m. Therefore, the speed of the wave is given by v = fλ = 50 x 0.008 = 0.4 m/s.