tolong dong plis bantuin aku ya

U5= 30
U12 = 16

(12-5)b= 16-30
7b = -14
b = -2
a+4b= 30
a+ -8= 30
a= 38

Un= bn+(a-b)
U30=-2.30+40
=-60+40
= -20

Sn= n/2 . (a+Un)
S30 = 30/2 . (38-20)
= 15.18
= 270