Globex Investment Capital Corporation owns six companies that have the following estimated returns $($in millions of dollars$)$ if sold in one of the next $3$ years:

$\backslash$table$[[,\backslash$table$[[$Year Sold$],[($Estimated return, $$1,000,000$s$)]]],[$Company$,1,2,3],[1,14,18,23],[2,9,11,15],[3,18,23,27],[4,16,21,25],[5,12,16,22],[6,21,23,28]]$

To operating funds, the company must sell at least $$24$ million worth of assets in year $1,$$$28$ million in year $2,$ and $$32$ million in year $3.$ Globex wants to develop a plan for selling these companies during the next $3$ years to maximize return.

$($a$)$ Formulate an integer programming model for this problem.