How to solve RMSE? not just answer!

please solve RMSE

\begin{tabular}{|l|r|r|} \hline Period & Time & \multicolumn{1}{|c|}{ Sales Forecast } \\ \hline 2017Q1 & 41 & -2201961.671 \\ \hline 2017 Q & 42 & -2201902.951 \\ \hline 2017 Q4 & 43 & -2201844.231 \\ \hline \end{tabular} \begin{tabular}{|l|l|r|} \hline Period & Time & Sales Forecast \\ \hline 2017Q1 & 41 & -2201961.671 \\ \hline 2017Q2 & 42 & -2201902.951 \\ \hline 2017 Q3 & 43 & -2201844.231 \\ \hline \end{tabular} A year later, Barbara gives you a call and tells you that the actual sales for the four quarters of 2017 were: Q1=334,271, Q2=328,982,Q3=317,921, and Q4=350,118. How accurate was your model? What was the mean absolute percentage error (MAPE)