last week Andrew bought 6 pounds of zucchini and 3 pounds of tomatoes for $10.05 at a farm stand. This week he bought 5 pounds of zucchini and 4 pounds of tomatoes, at the same prices, for $9.08.

So, 2x + 4x, is added to get 6x, because they are like terms, because they both have the same variable.  Then, we subtract 4y-2y= 2y. Then the answer would be 6x plus 2y. 
(Note: when you simply like therms they need to have the same variable to same power if they are addition of subtraction. Also, a term includes the negative or positive sign. And when all the work is finished and you need to put together your answer, always add, unless one of them is negative)

Steps:

=(2x+4x)+(4y-2y)
=6x+2y