So the linear equation is y =-2/3x+7,
According to Figure (2), the vertex of the parabola is (6, 1), which passes through point (3, 4).
So the equation of parabola is: y = 1/3 (x-6) 2+ 1,
Therefore, the sales price in March was 5 yuan/kg, the cost was 4 yuan/kg, and the profit per kg was 1 yuan.
In April, the price was (13/3) yuan/kg, the cost was (7/3) yuan/kg, and the profit per kg was 2 yuan.
The sales price in May was (1 1/3) yuan/kg, the cost was (4/3) yuan/kg, and the profit per kg was (7/3) yuan.
In June, the sales price was 3 yuan/kg, the cost 1 yuan/kg, and the profit per kg was 2 yuan.
Therefore, if you sell this vegetable in May, the profit per kilogram will be the biggest.