Sidney has 3 adult cats * 1 can of food = <<3*1=3>>3 cans of food per day.
Sidney's adult cats will need 3 cans * 7 days = <<3*7=21>>21 cans for the week.
Sidney has 4 kittens * 3/4 cup of food = <<4*3/4=3>>3 cans of food per day.
Sidney's kittens will need 3 cans * 7 days = <<3*7=21>>21 cans for the week.
Sidney will need to buy 21 cans + 21 cans - 7 cans = <<21+21-7=35>>35 cans.
#### 35