What is b=b i got that on a answer in khan academy it was Model with one-step equations and solve

Select a graph that have an equation with a < 0

If the speed of an electromagnetic wave is 100,000 cm/s and its wavelength is 100 cm, what is its frequency? a) 100,000 cycles per second (hz) b) 1 × 10-3 cycles per second (hz) c) 10,000,000 cycles per second (hz) d) 1,000 cycles per second (hz)

Can someone me with this problem i don't understand i need