Question 1011296
n is for the index of a term.
First term is {{{a[1]+(1-1)*d=a[1]+0*d=a[1]}}}.
Term at n=3 is {{{a[1]+(3-1)*d=a[1]+2d}}}.
The index for your last term of the sequence is unknown n, so it is  {{{a[1]+(n-1)d}}}, and is given as the expression;
{{{highlight_green(9x-z=a[1]+(n-1)d)}}}, and you do not know x, {{{a[1]}}}, n, d.


You want to find the value for z, yet the other needed quantities for x, {{{a[1]}}}, n, and d are still unknown.


The term for index of 3 is  {{{a[1]+2d=25}}}.  You can use this to find a formula for either {{{a[1]}}} OR  for d.  You will still have three unknown, undetermined variables.  Do you understand the difficulty here?


What is the entire problem description and question?