Ответ:

PascalABC.NET

begin

 var index_last_negative := 0;

 var c := ArrRandom(15, -99, 99);

 c.Println;

 foreach var number in c do begin

   if number < 0 then index_last_negative := c.IndexOf(number);

 end;

 index_last_negative.Println;

end.

Объяснение:

===== PascalABC.NET =====

begin

   var c := ArrRandom(13, -99, 99);

   c.Println;

   Print(c.FindLastIndex(p -> p < 0) + 1)

end.