I need help with questions $15-20!$ All of them

CozyforU is a manufacturer of down jackets. Its director of sales wants to develop seasonal forecast of sales that accounts for any trend or seasonality. The following table shows sales $($in millions of units$)$ of one product model for the past $16$ seasons.

Year $1234$

Winter $97\mathrm{.}1596\mathrm{.}2596\mathrm{.}1095\mathrm{.}35$

Spring $56\mathrm{.}3555\mathrm{.}9056\mathrm{.}0557\mathrm{.}25$

Summer $35\mathrm{.}0536\mathrm{.}1035\mathrm{.}3534\mathrm{.}90$

Fall $67\mathrm{.}9070\mathrm{.}0067\mathrm{.}6067\mathrm{.}45$

Question $15$: Calculate the slope of the least square regression line starting with Winter of Year $1$ as period $1$ and ending with Fall of Year $4$ as period $16($for example, Fall of Year $1$ will be period $4$ while Winter of Year $4$ will be period $13).$ If your answer is not an integer, provide at least three decimal places, eCalculate the intercept of the least square regression line starting with Winter of Year $1$ as period $1$ and ending with Fall of Year $4$ as period $16($for example, Fall of Year $1$ will be period $4$ while Winter of Year $4$ will be period $13).$ If your answer is not an integer, provide at least three decimal places, e$.$g$.,7\mathrm{.}500..$g$.,7\mathrm{.}500.$

Question $16$: Calculate the intercept of the least square regression line starting with Winter of Year $1$ as period $1$ and ending with Fall of Year $4$ as period $16($for example, Fall of Year $1$ will be period $4$ while Winter of Year $4$ will be period $13).$ If your answer is not an integer, provide at least three decimal places, e$.$g$.,7\mathrm{.}500.$

Question $17$: Use the regression results to calculate the seasonal index for each season of each year. From the indices for all sixteen seasons, calculate the average seasonal index for each season.

What is the Average Seasonal Index for Spring? If your answer is not an integer, provide at least three decimal places, e$.$g$.,7\mathrm{.}500.$

Question $18$: Use the regression results to calculate the seasonal index for each season of each year. From the indices for all sixteen seasons, calculate the average seasonal index for each season.

What is the Average Seasonal Index for Fall? If your answer is not an integer, provide at least three decimal places, e$.$g$.,7\mathrm{.}500.$

Question $19$: Using the regression results and the average seasonal indices, derive a sales forecast $($in millions of units$)$ for each season of Year $5$ that accounts for both trend and seasonality.

What is the Final Forecast for Winter of Year $5?$

Question $20$: Using the regression results and the average seasonal indices from part, derive a sales forecast $($in millions of units$)$ for each season of Year $5$ that accounts for both trend and seasonality.

What is the Final Forecast for Summer of Year $5?$