Question 1092307
<pre>
{{{S[n]}}}{{{""=""}}}{{{expr(n/2)(2a[1]+(n-1)d^"")}}}

Substitute n = 16, a<sub>1</sub> = -21

{{{S[16]}}}{{{""=""}}}{{{expr(16/2)(2(-21)+(16-1)d^"")}}}

Substitute S<sub>16</sub> = -288.

{{{-288}}}{{{""=""}}}{{{(8)(2(-21)+(16-1)d^"")}}} 

Now simplify that and solve for d.

When you find d, substitute it and a<sub>1</sub> = -21

into 

{{{a[n]}}}{{{""=""}}}{{{a[1]+(n-1)d}}}

And simplify the right side:

You finish.

Edwin</pre>