A magnetic field does nothing to a charge that is sitting still. Set that charge moving and the field pushes on it — sideways, at right angles to both the motion and the field. The size of that push is $F = q v B \sin\theta$, read aloud F equals q v B sine theta.

Every letter, in words. $F$ is the magnetic force in newtons. $q$ is the charge in coulombs — quoted in microcoulombs on the bench, so convert. $v$ is the charge's speed in metres per second. $B$ is the magnetic flux density — the field strength — in tesla, and one tesla is a very strong field: a fridge magnet manages a few thousandths of one, an MRI bore runs to three. And $\theta$ — theta — is the angle between the velocity and the field, in degrees. You will solve this for $F$, for $B$, and for $v$.

Two things worth carrying out of this lesson. First, $v$ is in the numerator with nothing to soften it, so $v = 0$ gives $F = 0$, full stop. A stationary charge in the strongest magnet ever built feels precisely nothing. Second, the sine runs the other way from the incline's: here $\sin 90^\circ = 1$, so a charge crossing the field SQUARE-ON feels the maximum force, and a charge running ALONG the field lines at $\theta = 0$ feels none at all. Anchor it with those two limits and you will never have to remember which trig function it is.