Question 1123336
{{{ L = a+(n-1)*d }}}
Subtract a from both sides:
{{{ L-a = (n-1)*d }}}
Divide by n-1 on both sides:
{{{  (L-a)/(n-1) = d }}}

Re-writing with d on left hand side:
{{{ d = (L-a)/(n-1) }}}    with the restriction {{{ n <> 1 }}}