Light is a wave, and every wave — light, sound, the ripple crossing a bathtub — obeys one sentence: $v = f\lambda$. Read aloud: v equals f lambda. Three letters, three jobs. $v$ is the wave's speed, in metres per second. $f$ is its frequency, in hertz — how many whole waves pass a fixed point each second. $\lambda$ is its wavelength, in metres — the length of one complete wave, crest to crest. That last symbol is the Greek letter lambda, said “LAM-da”. Whichever of the three the question withholds is the one you solve for.

For light in a vacuum the speed is not yours to choose: $c = 3.00 \times 10^8\ \mathrm{m/s}$, the universe's posted limit, identical for red light, radio and gamma rays. So $v = f\lambda$ becomes $c = f\lambda$, and frequency and wavelength are locked together on a see-saw: push one up and the other must come down. There is no light anywhere that breaks that trade.

Its constant companion is $T = \dfrac{1}{f}$ — T equals one over f — where $T$ is the period, the number of seconds one cycle takes, and $f$ is that same frequency in cycles per second. They are reciprocals, and the units announce it out loud: seconds per cycle against cycles per second. Neither is more fundamental; they are one fact read from either end.

Now the prefixes, because light is small and fast and the exam knows it. A nanometre is $10^{-9}\ \mathrm{m}$, and visible light runs from about 400 nm (violet) to 700 nm (red). A terahertz is $10^{12}\ \mathrm{Hz}$, and that same visible band runs roughly 430 to 750 THz. Cross those bridges BEFORE the numbers enter the formula, never after — a prefix crossed late is a prefix crossed wrong.