Another way to think of this is that you multiply both the numerator and the denominator of the RHS by the same value \(\displaystyle \left({b \over nx}\right)\) which makes the denominator equal to 1, so it can be discarded and we are left only with the numerator (which is now a fraction).

\(\displaystyle {n \over {nx \over b}} = {n \times {b \over nx} \over {nx \over b} \times {b \over nx}} = {{nb \over nx} \over 1} = {nb \over nx} = {b \over x}\)