The raisins cost $3.00 a box while the nuts cost $6.00 a tin.
The graph in the picture provide to us, in the shaded region, the relation between the number of boxes of raisins and the tins of nuts that can be buyed without spend more than $12.00.
We can check in the graph that the point representing a mixture consisting of 3 boxes of raisins and 1 tin of nuts is out of the shaded region.
Therefore, Randy can't make this mixture without spend no more than $12.00