Answer
$p=\frac{\sqrt{ KE^2+2(KE)(mc^2)}}{c} $
Work Step by Step
Start with equations 26–6a and 26–9.
$$E=KE+mc^2$$
$$(pc)^2=E^2-(mc^2)^2$$
Now combine them, substituting E from the first equation into the second equation.
$$(pc)^2= (KE+mc^2)^2-(mc^2)^2$$
$$(pc)^2= KE^2+2(KE)(mc^2) $$
$$p=\frac{\sqrt{ KE^2+2(KE)(mc^2)}}{c} $$