The equation relating a wave's propagation speed to its frequency and wavelength:
v = fλ
v is the progatation speed, f is the frequency, and λ is the wavelength.
Given values:
For an EM wave in a vacuum, v = c ≈ 3×10⁸m/s
λ = 0.07m
Plug in the given values and solve for f:
3×10⁸ = 0.07×f
f = 4.3×10⁹Hz