The relationship between the sales unit price of vegetables and the sales month is shown in (1).

The relationship between (1) selling price y and month x is Y=KX+b, and the equations are obtained from (3,5) and (6,3):

5=3K+b，

3=6K+b，

Solution: K=-2/3, b=7,

∴Y=-2/3X+7。

⑵ Let the relationship between the cost w and the month x be w = a (x-6) 2+ 1, (3,4) and then get the equation:

4 = A (3-6) 2+ 1, the solution is: a= 1/3,

∴w= 1/3(x-6)^2+ 1= 1/3x^2-4x+ 13，

Income p = y-w =-2/3x+7-1/3x2+4x-13.

=- 1/3X^2+ 10/3X-6

=- 1/3(X-5)^2+7/3，

∴ When X=5 and P max =7/3,

∴ Probably the biggest gain.