tolong bantu jawab pke cara ya, trimakasih (:

#a+a+b+a+2b=-9
3a+3b=-9
#a+2b+a+3b+a+4b=15
3a+9b=15

3a+3b=-9
3a+9b=15
6b=24
b=4

3a+3b=-9
3a+12=-9
3a=-21
a=-7

S5= n/a (2a + (n-1)b)
=5/2 (2(-7)+(4)4)
=5/2 (4)
=10