Fungsi f(x,y) mencapai maksimum di (2,3) jika... Jawaban benar (C), mohon dibantu caranya

(2,3) adalah titik potong dari 3x + y ≤ 9 dan x + 2y ≤ 8
Maka berlaku : m1 ≤ mf ≤ m2
ax + by = c => m = -a/b
3x + y ≤ 9 => m1 = -3/1 = -3
x + 2y ≤ 8 => m2 = -1/2
f(x,y) = cx + 4y => mf = -c/4

m1 ≤ mf ≤ m2
-3 ≤ -c/4 ≤ -1/2 ======> kali 4
-12 ≤ -c ≤ -2 ========> kali -1
12 ≥ c ≥ 2 => 2 ≤ c ≤ 12