Can you explain how to do this question through Excel, or if its simple enough, explain through written math, Thank you!

Centerville Bikes and Stuff $($CBS$)$ sells motorcycles and accessories. The number of helmets sold by CBS per week for the past six weeks follows.

Using the naive method $($most recent value$)$ as the forecast for the next week, compute the following measures of forecast accuracy.

$($a$)$ mean absolute error

$"$ MAE $"=$

$($b$)$ mean squared error

MSE $=$

$($c$)$ mean absolute percentage error $($Round your answer to two decimal places.$)$

MAPE $=$

$\%$

$($d$)$ What is the forecast for week $7?$