posted by samuel .
need help with tthe first question on this page!!! please
They are asking for a complete writeup of the solution in the form of a statement, assumptions, method, analysis and conclusions. I do not have tiome to do that for you.
The velocity selector has the B field perpendicular to the page, with v*B = E. The Electric force balances the magnetic force. The E field goes from positive to negative plate. Use the right hand rule to get the B direction there.
In the detection chamber,
m v^2/r = B v e, with a different value of B, but the same v as above. Use that to solve for the mass M