a, b and d all represent the combined area of the gardens
(6*7) + (6*x) is equivalent to 6(7+x) and 42 + 6x
Check:
(6*7) + (6*x)
42 + 6x
6(7+x)
42 + 6x
42 + 6x
William chose C because he added the length and width of the vegetable garden (7 + 6 = 13), and then added the length and width of the flower garden (6 + x = 6 + x).
To find the area you multiply width times height, here for the bigger rectangle, to find the area you multiply 7*6 and then add that with 6 * x because those are the width and height of the smaller rectangle.
Second part
I believe that Wiliam chose C because he accidentally thought that adding everything was correct. He added 7+6 to get 13 for the bigger rectangle and then for the smaller rectangle decided that to find the area you would add 6 and x.
