Calculate Intersections

• Draw: \draw [name path=l1] (-1,-1) -- ( 1,1);
\draw [name path=l2] ( 1,-1) -- (-1,1);
  
• Find Intersection:
  \path [name intersections={of=l1 and l2,by=X}];
• Draw it:
  \filldraw (X) circle(1pt) node [right] {$X$};