Answer
$140 MeV/c^2$.
Work Step by Step
Use equations 26–6a and 26–9.
$$E^2=(KE+mc^2)^2=p^2c^2+m^2c^4$$
$$KE^2+2(KE)mc^2+m^2c^4=p^2c^2+m^2c^4$$
$$KE^2+2(KE)mc^2=p^2c^2$$
Solve for the mass.
$$m=\frac{p^2c^2-KE^2}{2(KE)c^2}$$
$$=\frac{(121MeV/c)^2c^2-(45MeV)^2}{2(45MeV)c^2}=140 MeV/c^2$$
