What is the acceleration of a proton moving with a speed of 7.0 m/s at right angles to a magnetic field of 1.7 t ?

We know that a charge moving in a magnetic field is subject to the force:
F = q · v · B

But we also know that:
F = m · a

Therefore, it must be:
m · a = q · v · B

And solving for a:
a = q · v · B / m

Recall that for a proton:
q = 1.6×10⁻¹⁹ C
m = 1.673×10⁻²⁷ kg

Now, you can find:
a = 1.6×10⁻¹⁹ · 7.0 · 1.7 / 1.673×10⁻²⁷
   = 1.14×10⁹ m/s²

Hence, the acceleration of the proton is 1.14×10⁹ m/s².