$b.$ Test to see whether the coefficients you have estimated are statistically different from zero, using a $95$ percent confidence level and a one$-$tailed test.

c$.$ What percentage of the variation in sales is explained by this model?

d$.$ Use this model to make a sales forecast $($SF$1)$ for $2017$Q$1$ through $2017$Q$4,$ given the previously forecast values for unemployment $($NRURF$)$ and income $($INCF$)$ as follows:

$\backslash$table$[[$Period$,$Sales,Inc,NRUR,Time$],[$Mar$-07,72962,218,8\mathrm{.}4,1],[$Jun$-07,81921,237,8\mathrm{.}2,2],[$Sep$-07,97729,263,8\mathrm{.}4,3],[$Dec$-07,142161,293,8\mathrm{.}4,4],[$Mar$-08,145592,318,8\mathrm{.}1,5],[$Jun$-08,117129,359,7\mathrm{.}7,6],[$Sep$-08,114159,404,75,7]]$