tolong kerjain dong, udah mentok gabisa

y = 5
x + 5 = 20
x = 15
(x,y) = (15,5)

x = 5
5 + y = 20
y = 15
(x,y) = (5,15)

f(15,5) = 3(15) + 4(5) = 45 + 20 = 65
f(5,15) = 3(5) + 4(15) = 15 + 60 = 75
nilai minimum