$ $($Chapter $15).$ As a regional sales manager, you are asked by the CEO to perform a multiple linear regression analysis on the factors that affect the sales. The purpose is to examine the significance level of the factor and predict future sales accurately. After research, you have found that your time with company $(x1,$ market potential of the product $(x2),$ advertising $(x3,$ market share of the product $(x4),$ and market share change $(x5)$ are likely to impact the sales $(y).$

$($a$)$ Write the multiple linear regression equation below.

$($b$)$ What is the relationship between time with company $($xl$)$ and the sales $($y$)?$ How significant is this relationship and why?

$($c$)$ What is the relationship between market share change $(x)$ and the sales $($y$)?$ How significant is this relationship and why?