pulse dispersion calculation

Hello, I am trying to solve the following:

A fiber suffers from a pulse dispersion problem. Given that the fiber has a pulse dispersion of 5ns/km and the signal pulse width in the fiber (tw) is 20ns, determine the maximum bit rate that it can achieve without bit errors due to intersymbol interference if the fiber length is 12km.

My attempt:
using the forumula:
Total pulse dispersion (ps) = dispersion value(ps/nm.km) * spectral width of light source (delta wavelength nm) * length of fiber(km)

? = 5ns/km * 20ns * 12km

But this is the first time I have been exposed to this so am unsure if I am completing the formula correctly based on info. provided. Can someone please offer some help ?

Thanks kindly.