The number is 21 because 2 is 1 doubled, and 2+1 is 3.
So you have to find the relationship between the pumpkin diameter (y) and the number of weeks passed (x) in a table, lets do it taking into account that the equation modeling such behaviour is:
y = 2x + 6, where x is the number of weeks and plus original 6 cm
x y
week diameter
0 6
1 8
3 12
5 16
10 26
substitute the x and y values in the equation to see how they fit into it
Amounttotal=amount per day times number of days
200=17.1 times d
divide both sides by 17.1
11.695=d
about 12 days
9 times 9 = 81 6 times 6 =36 so 36 plus 18