First term (a) = -15
common difference(d)= -9-(-15)
= -9+15=6
So,
12th term(a12)= a +(n-1)d
= -15+(12-1)×6
= -15 + 11×6
= -15+66
= 51
Cid Sam is a way to remember
C -control S - SAME (keep the same)
I -independent A- alter
D-depend M- measure
independent- number of downloads
Dependant - money spent
IdentityProperty of Addition OR Zero Property