Wait 0 weeks: 100 * 500 = 50000
Wait 1 week: 110 * 475 = 52250
Wait 2 weeks: 120 * 450 = 54000
You can actually just calculate them all out and get to the maximum pretty quickly, but as for equation, let be the number of weeks the grower waits, and let be profit. So the equation is
This is quadratic; from your post it seems that you know where to go from here.