Answer
$E_K=\frac{-2mc^2\pm\sqrt{4m^2c^4+4p^2c^2}}{2}$
Work Step by Step
$E^2=p^2c^2+m^2c^4$
$E=E_K+mc^2$
$E^2=E_K^2+2E_Kmc^2+m^2c^4$
$p^2c^2+m^2c^4=E_K^2+2E_Kmc^2+m^2c^4$
$E_K^2+2E_Kmc^2-p^2c^2=0$
$E_K=\frac{-2mc^2\pm\sqrt{4m^2c^4+4p^2c^2}}{2}$
When mass is $0$
$E_K=pc$
