[ART]

Вот, вроде правильно:

var
xp,yp:real;

procedure Tform1.Lines(x1,y1,x2,y2,x3,y3,x4,y4:real);
var
k1,k2:real;
m1,m2:real;
x,y:real;
begin
k1:=(y2-y1)/(x2-x1);
k2:=(y4-y3)/(x4-x3);
m1:=-k1*x1+y1;
m2:=-k2*x3+y3;
x:=(m2-m1)/(k1-k2);
y:=(k1*m2-k2*m1)/(k1-k2);
xp:=x;
yp:=y;
end;