​cant do thiss

Not that hard, ISG........  

If the dog eats 7cans of food in 3 days....he eats

7 cans / 3 days  =   (7/3) cans each day 

So in  3 + d   days.....he eats.......  7 + (7/3)d     cans    [ answer K ]

To get a feel for this...........  let d  = 3

So  he eats   7 cans the first 3 days +  (7/3)(3)  =    7 + 7  =  14 cans in a total of 6 days

LOL!!!!......I often do that, too....!!!!!