If the speed of a wave doubles while the frequency remains the same, it means

Velocity of a wave is describe as

velocity = Frequency * Wavelength

Mathematically

v = fλ

Hence, Frequency, F = v / λ

Wavelength λ = v/f

So, if the frequency is kept constant, wavelength of the wave becomes directly proportional to velocity of the wave.

And this implies that, as the speed double, the wavelength is double.