Answer
Please see the work below.
Work Step by Step
We know that the time of flight of the protons is given as
$t=\sqrt{\frac{2y}{g}}$
We plug in the known values to obtain:
$t=\sqrt{\frac{(2)(1.2\times 10^{-6})}{9.8}}$
$t=4.94\times 10^{-4}s$
The speed of the protons is
$v=\frac{x}{t}$
We plug in the known values to obtain:
$v=\frac{1700}{4.949\times 10^{-4}}=3.4\times 10^6\frac{m}{s}$
