a charged particle traveling follows a straight path through crossed electric and magnetic fields if the strength of the magnetic field what is the strength of the magnetic field?

When a charged particle is moving in a straight line, means undeflected in a crossed magnetic and electric field, it means the net force on the charged particle is zero.

magnetic force is balanced by the electric force

q v B = q E

Where, q be the charge on the particle, v be the velocity, B be the strength of magnetic field, e be the strength of electric field.

So, B = E / v