a. The expression given in the question is
c = 6x + (900000/x)
c = (6x^ +900000)/x
I hope that this is the expression that you were looking for.
b. When x = 240, then
c = [6 * (240)^2 + 900000]/ 240
= (6 * 57600) + 900000/240
= (345600 + 900000)/240
= 5190
From the above deduction, it can be concluded that the cost of ordering and storing is 5190. I hope the procedure is clear enough for you to understand.