The correct answer is v_y + Eed/m_ex v_x .
Any vector's magnitude, represented by its unit and pointing in the direction of the vector, is its length. As a result, the acceleration's magnitude is equal to the length of the acceleration vector and it is pointed in the same direction.
let the magnitude charge of the electron =e = 1.6 x 10^-19 C
me= mass of electron = 9.11 x 10^-31
force on the electron = Ee
Magnitude of acceleration a= Ee/m_e
In x direction there is no field --> no force---> no acceleration
d = v_xt
t = d/v_x
the y-component of electron's velocity when its x coordinate has changed by a distance d v_y' = v_y + at = v_y + Eed/m_ex v_x
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