The mass of the proton is:
[tex]m_p = 1.67 \cdot 10^{-27} kg[/tex]
and the mass of the antiproton is exactly the same, so the total mass of the two particles is [tex]2m_p[/tex].
In the annihilation, all the mass of the two particles is converted into energy, and the amount of this energy is given by Einstein's equivalence between mass and energy:
[tex]E=Mc^2[/tex]
where M is the mass converted into energy and c is the speed of light. In this example, [tex]M=2m_p[/tex], therefore the energy released is
[tex]E=2m_p c^2 = 2 (1.67 \cdot 10^{-27} kg)(3\cdot 10^8 m/s)^2=3 \cdot 10^{-10}J[/tex]
