Consider the following time series.

Period

$1$

$2$

$3$

$4$

$5$

$6$

$7$

$8$

Demand

$8$

$9$

$7$

$10$

$13$

$14$

$12$

$15$

$$a$)$ Using a trend$$ projection, forecast the demand for Period $9.$

$$b$)$ Calculate the MAD for this forecast.