You have two fertilizers to use. Draw at least one iso-cost line. Draw a circle or an arrow pointing out the optimal point. Because you're using the graph method, these applets will call your answer correct if you are within 0.5 of the right answer. By the way, to get full credit, your graph needs to show the constraints, at least one iso-cost line, and the point representing the optimal combination of fertilizers.
Some people have trouble graphing the potash constraint.
If you got that right, this one should be right, too, but let's check anyway. Now you're told that you must buy at least 6 bags of the first fertilizer. This adds a constraint, another line to your graph. There are four shadow prices, one for each constraint, so you need four sentences, one for each shadow price, to answer this question. Depending on which software you are using and what options you picked, the shadow prices may be called Lagrange Multipliers or Opportunity Costs.