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Did you even try to finish grade school? This problem is child's play!
Magnetic force law: F = qv x B (cross product); F is the force pulling the proton to the earth, q is the charge of the proton, v is the velocity of the proton, and B is the magnetic field. Since we are working with objects near the surface of the earth, the cross product is simply a product.
Now, what is F? The force pulling the proton, which is moving at a constant velocity, into a circular rotation. Wait a minute, that sounds like centripetal force! F=mv^2/r, as we all know, and that F equals the F from the previous equation. Thus, F = qvB = mv^2/r.
Well, wait, now we have v on both sides. Since we can divide both sides by the same value without damaging the equality, we can get qB = mv/r. Now we can solve for v to find the speed of the proton:
v = qBr/m
q = 1.602*10^-19 As
B = 4*10^-8 kg/As^2
r = (radius of earth) +900km = 7.2675*10^6m
m = 1.6726*10^-27kg
v = (1.602*10^-19)(4*10^-8)(7.2675*10^6)/(1.6726*10^-27) (kgmAs)/(As^2kg) = 2.784*10^7 m/s
See? Simple.
