The answer is 5.88 · 10⁻⁷ m.
To calculate this we will use the light equation:v = λ · f,
where:
v - the speed of light (units: m/s)
λ - the wavelength of the ray (units: m)
f - the frequency of the ray (units: Hz = 1/s since Hz means cycles per second (f=1/T))
It is given:
f = 5.10 · 10¹⁴ Hz = 5.10 · 10¹⁴ 1/s
v = 2.998 · 10⁸ m/s
λ = ?
If v = λ · f, then λ = v ÷ f:
λ = 2.998 · 10⁸ m/s ÷ 5.10 · 10¹⁴ 1/s
= 0.588 · 10⁸⁻¹⁴ · m
= 0.588 · 10⁻⁶ m
= 5.88 · 10⁻⁷ m