D itu jawaban dari guru saya, tpi saya tidak tau jalanny:'

U5 = a + 4b = 30
U12 = a + 11b = 16 -
-7b = 14
b = 14/-7 = -2
a + 4b = 30
a + 4(-2) = 30
a - 8 = 30
a = 30 + 8 = 38

S30 = 30/2 (2(38) + (30 - 1) -2)
= 15 (76 + 29(-2))
= 15 (76 - 58)
= 15 (18) = 270

Maaf klo salah :)

Diketahui
u5 = 30
u12 = 16

u12 - u5 = 16 - 30
7b = -14
b = -14 ÷ 7
b = -2

u5 = 30
a + 4b = 30
a + 4(-2) = 30
a - 8 = 30
a = 30 - (-8)
a = 30 + 8
a = 38

Sn = n/2 (2a + (n-1) b)
S30 = 15 (2(38) + 29(-2))
S30 = 15 x (76 - 58)
S30 = 15 x 18
S30 = 270