It looks to me like you have two variables- x, the number of days factory 1 is run, and y, the number of days factory 2 is run. You will then produce x+ 2y units of A grade steel and 3x+ 2y units of grade B steel. The cost will be \(\displaystyle 5000x+ $6000y\). The problem wants you to minimize the cost so that is the objective function. Minimize that subject to the constraints that \(\displaystyle x+ 2y\ge 80\) and \(\displaystyle 3x+ 2y\ge 140\).