Alight wave travels at a speed of 3.0 × 108 meters/second. if the wavelength is 7.0 × 10-7 meters, what is the frequency of the wave?

c = f*L 

If you cant remember this just consider the units of each (denoted [c] is units of c): 

[c] = meters/second, [f] = 1/seconds or Hertz, [L] = meters 

So [c] = [f*L], an easy way to remember it. 

So to get the frequency, just solve for f: 

f = c/L = (3.00 x 10^8)/(5.93 x 10^-7) m/s = 5.05902192 × 10^14 m/s ~ 5.06 x 10^14 m/s 

58750.08068 is the answer i think