HI!
instead of just giving you the answer, i will help you to understand it in simple terms.
As electromagnetic waves (read: light) move forwards it 'vibrates' up and down. The frequency of light tells us how many up-down vibrations it makes each second (1hz = 1 up-down vibration per second & 1mhz = 1 million up-down vibrations per second). Each individual vibration is called one wave.
Since we also know how many meters light travels in 1 second (290,000,000 meters), we can easily see that (in the case of this specific question) every '290,000,000' meters, there are '835,000,000' vibrations or waves.
It's then just a simple division to work out what is the length of just one wave! - which is what we call the wavelength of light at that frequency.
290,000,000 / 835,000,000 = 0.34m (or 34cm)
