Question

a1=10 d=-9 n=15 someone help ?

Answer 1

help with what ?

Answer 2

arithmetic series.....

Answer 3

You got almost everything ...
a =First term .
d = difference between every two successive terms .
n = Number of the turn .
Just Apply the rule and fill in .

Answer 4

The formula you need is \[a_{n}=a_{1}+(n-1)d\] and like @Eyad said, plug and chug.