Show that the speed v of a particle of total energy E and momentum p is given by the following equation.

$$v = \frac{{p{c^2}}}{E}$$

Determine, using the answer in (a), the speed of a particle whose rest mass is zero.

combined use of p = γmv and E=γmc²;

eliminate the mass and gamma factor by, for example, dividing to get $\frac{p}{E} = \frac{v}{{{c^2}}}$;
to get the result
Accept going backwards from given result to reach correct formulae.

for a particle with zero rest mass, E=pc;
and so $v = \left( {\frac{{p{c^2}}}{{pc}} = } \right)c$;

Award [1] for “zero rest mass particles are photons and so v = c”.