The answer is A for the correct answer
Since we are asked to find the rate of change from Monday to Wednesday, we are dividing by 2, since Monday is day 1 and Wednesday is day 3. 3 - 1 = 2.
1800 + 500 = 2300 calories consumed Tuesday.
2300 - 100 = 2200 calories Wednesday.
(2200 - 1800)/2 = 400/2 = 200 calories
You just need to plug those expressions inside the formula: it doesn't matter if they're expressions involving a variable instead of plain numbers: the formula becomes

If you want, you can simplify it by expanding the square and then multiply the two parenthesis:

The formula for surface area is SA = 2 (wl + hl + hw). Plugging in the numbers that we know, we get SA = 2 (100 + 10h +10h). We know that the crust accounts for half of the surface area.
The surface area was not provided so this is the closest I can go without the provided surface area. I hope this helps you :)