If you notice the picture below, the amount of fencing, or perimeter, that will be used will be 3w + 2l
now

solve for "w", to see what critical points you get, and then run a first-derivative test on them, for the minimum
notice the

so. you can pretty much skip that one, though is a valid critical point, the width can't clearly be 0
so.. check the critical points on the other
8 / 2 = 45 / x....8 servings to 2 cups = 45 servings to x cups
cross multiply
(8)(x) = (2)(45)
8x = 90
x = 90/8
x = 11 1/4 (or 11.25) cups of flour are needed <==
Answer:
a) 26.2
b) 13.8
Step-by-step explanation:
You add all side lengths, which are given.
a) 9+4.1+9+4.1 = 26.2
b) 3.6+3+7.2 = 13.8
The answer to this should be 1.51