documentclass[tikz]{standalone}
usetikzlibrary{math}

begin{document}
begin{tikzpicture}
draw[gray!40!] (-0.5,-0.5) grid (6.5,4.5);
tikzmath{
    int x1; int x2;
    x1 = 0; x2 = 2;
}
draw[black,thick] (x1,2)  -- (x2,2);
foreach X in {x1,x2}{
    X=X+3;
}
draw[black,thick] (x1,2)  -- (x2,2);
end{tikzpicture}
end{document}