problem can be solved using the equation
c = freq x wavelength, where c is the speed of light, which is 3.00 x 10^8 m / s
freq = c / wavelength
freq = (3.00 x 10^8m /s) / (2.40 x 10^-5 cm x 1 m /100cm)
freq = (3.00 x 10^8 m/s) / 2.40 x 10^-7m
freq = 1.25 x 10^15/s 1 / s = 1Hz
freq = 1.25 x 10^15Hz
