First, let's calculate the electric force, using the formula below:
[tex]\begin{gathered} F_e=\frac{KQq}{d^2}\\ \\ F_e=\frac{9\cdot10^9\cdot(22.78\cdot10^{-6})^2}{4.22^2}\\ \\ F_e=262.26\cdot10^{-3}=0.26226\text{ N} \end{gathered}[/tex]Now, let's use the second law of Newton to find the acceleration:
[tex]\begin{gathered} F=m\cdot a\\ \\ 0.26226=1.21\cdot a\\ \\ a=\frac{0.26226}{1.21}\\ \\ a=0.217\text{ m/s^^b2} \end{gathered}[/tex]