After leaving the gun no forces accelerate in horizontal direction. So you can use the following equations: $$\begin{equation}x(t)= v_0\cdot t\end{equation}$$ $$\begin{equation}v= v_0\end{equation}$$ $$\begin{equation}a=0\end{equation}$$

After leaving the electron gun no forces accelerate in horizontal direction. So you can use the following equations: $$\begin{equation}x(t)= v_0\cdot t\end{equation}$$ $$\begin{equation}v= v_0\end{equation}$$ $$\begin{equation}a=0\end{equation}$$