The Star gold plated items are sold at the jewelry shop. The sales data during the last thirteen days as per table $1$:

Table $1$: The sales record of flower shop

Day

$1$

$2$

$3$

$97$

$4$

$5$

$6$

$7$

$8$

$9$

$10$

$11$

$12$

Sales

$87$

$95$

$95$

$98$

$99$

$89$

$93$

$96$

$93$

$97$

$94$

$99$

$13$

i$)$ Derive a least square linear forecasting equation for the above case.

ii$)$ Use your equation to forecast the trend value for the $18$th and $19$th day.