Question 332879
in an arithmetic sequence, 

the nth term is defined as :  {{{a[n]=a1+(n-1)*d}}}
therefore

given:  {{{a[1]=-6}}}, {{{a[n]= -23}}} and d = -1, substitute into {{{a[n]=a[1]+(n-1)*d}}} and solve for n

-23=-6+(n-1)*(-1)
-23+6=-(n-1)
17=n-1
18=n

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so,  {{{a[18]=-23}}} is the 18th term.