Question 1144046
each time you're adding 3 to the previous number.
the formula is therefore An = An-1 + 3
A1 = -15
A2 = -15 + 3 = -12
A3 = -12 + 3 = -9
A4 = -9 + 3 = -6
A5 = -6 + 3 = -3
etc.....