part of my program:
...
for x:=0 to p-1 do
for y:=0 to p-1 do
if f(x,y,p)=(k mod p) then
begin
writeln(x,' ',y);
halt(0);
end;
writeln('NO SOLUTION');
end.
{-----------------------------------}
f(x,y,p) = ( x^2 + y^2 ) mod p