Question

find the nth term of -15, -18 ,-21 ,....

Answer 1

-15, -18, -21............

This is an Arithmetic progression
First term = a = -15 and the common difference = -18-(15) = -3

nth term of an AP = a + (n-1)d
    
                             = -15 + (n-1)(-3)
 
                             = -15 -3n +3
 
                             = -12 - 3n

Answer 2

-24
these numbers go in 3 tables so
-3,-6,-9,-12(-15,-18,-21,-24)