They did not include the constraint for y ≤x+3 on the graph.
See attached picture with added constraint.
Using the 4 points that are given as the solution on the graph, replace t he x and Y in the original equation to solve and see which is the greater value.
Point (0,3) P = -0 +3(3) = 0+9 = 9
Point (1,4) P = -1 + 3(4) = -1 +12 = 11
Point (0,0) P = -0 + 3(0) = 0 + 0 = 0
Point (3,0) P = -3 + 3(0) = -3 + 0 = -3
The correct solution to maximize P is (1,4)
