Example - direction
Since the field is directed into the screen, a velocity directed up (+y) would experience a force to the left (-x) if it were a positive charge. Since we have an electron which has a negative charge, the magnetic force would be to the right (+x).
The target is to the right and above. The direction straight to the target is:
qtarget = tan-1 (y/x) = tan-1(42 cm / 25 cm) = 59.2o.
Because the force will push the electron to the right, we must aim 90o to the left, which means we must use the plus sign:
qv = qtarget +/- 90o = 59.2o + 90o = 149.2o .