Tolong di jawab yang benar ya plisss

33)
f(a) = 12 – 4(a)
-36 = 12 – 4a
4a = 48
a = 12

31)
a = 25
b = -3
Un = a + (n-1)b
U12 = a + 11b
= 25 + 11(-3)
= 25 – 33
= -11
Sn = n/2 (a + Un)
S12 = 12/2 (25 + (-11))
S12 = 6(14)
S12 = 84