Answer
$1.8\times10^{-19}m$
Work Step by Step
The kinetic energy of the protons is many times greater than their rest mass energy, so their energy is basically their KE, and E = pc. Use the equation for the de Broglie wavelength, $p=\frac{h}{\lambda}$.
$$E=pc=\frac{hc}{\lambda}$$
$$\lambda=\frac{hc}{E}$$
$$=\frac{(6.63\times10^{-34} J \cdot s)(3.00\times10^8m/s)}{(7.0\times10^{12}eV)(1.60\times10^{-19}J/eV)}$$
$$=1.8\times10^{-19}m$$
