The wave equation links speed, frequency and wavelength

Progressive Waves — AQA A-Level Physics

v = f\lambda

$v$: wave speedThe distance travelled by a wavefront per unit time. (m $s^{-1}$)
$f$: frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). (Hz)
$\lambda$: wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m). (m)

This applies to all waves: transverse and longitudinal, electromagnetic and mechanical.
For a wave of constant speed: as wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m). increases, frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). decreases and vice versa.
The speed of electromagnetic waves in a vacuum is $c = 3 \times 10^{8}$ m s$^{-1}$.

Examiner Tips and Tricks

You may see the wave equation written as $c = f \lambda$ where c is the wave speedThe distance travelled by a wavefront per unit time..
However, c is often reserved for the speed of light ($3 \times 10^{8}$ m $s^{-1}$).
Only electromagnetic waves travel at this speed, so use v for any speed that is not the speed of light.