The Dutch scientist Christiaan Huygens who first did the arithmetic found a value for the speed of light equivalent to 131000 miles per second. The speed of light is equal to the distance light travels d divided by the time it takes t. In the ensuing centuries a number of other scientists worked to determine the speed of light and using improved techniques came up with increasingly accurate calculations.
Since frequency and wavelength are inversely proportional velocity of light or c is constant.
Scientists have measured it by bouncing lasers back from objects and watching the way gravity acts on planets and all these experiments come up with the same figure. Your microwave has its wavelength listed somewhere and its easy to multiply the rest. The Dutch scientist Christiaan Huygens who first did the arithmetic found a value for the speed of light equivalent to 131000 miles per second. Gordon-Smith used a cavity resonator to calculate the speed of light by measuring its wavelength and frequency.