It seems you are talking about radio waves. The wavelength (40 m) multiplied by the frequency (7 million / second) must equal the speed of light (300 million meters/second). It seems that in this example numbers, either the wavelength or the frequency, or both, are not expressed with a great accuracy. For example, if 40 meters is exact, the frequency would be close to 7.5 MHz.
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Frequency (1/seconds) x Wave Length (meters) = Speed (meters/sec. or m/s)
It is a constant which is equal to the speed.
The speed of a wave is equal to its wavelength times its frequency. Since you are using SI units, the answer will be in meters/second.
The formula for a wave in this case is: speed = frequency x wavelength. Since Hz = 1/second, the answer will be in meter/second.
wave length and frequency are the product of the wave speed, so the wave speed is a constant variable and the other two are inversely proportional the wave length increases, as the frequency decreases