Question 362321
{{{(9-5)/(-1--5) = (5-9)/(n--1)}}},
{{{4/4 = -4/(n+1)}}},
{{{1 = -4/(n+1)}}},
{{{n+1 = -4}}},
n = -5.
On 2nd thought the value of n should have been clear just by inspection (how?)