documentclass[tikz, border=1cm]{standalone}
usetikzlibrary{intersections}
begin{document}
%
begin{tikzpicture}
    node[name=w, rectangle, fill=darkgray, minimum width=3mm, minimum height = 6cm]{};
    path[name path=wnw] (w.north west)--++(-6, 0);
    path[name path=wv] (w.west)--++(128:6);
    path [name intersections={of=wnw and wv, by={B}}];
    draw (w.west)--(B);
end{tikzpicture}
%
end{document}

documentclass[tikz, border=1cm]{standalone}
usetikzlibrary{intersections}
begin{document}
%
begin{tikzpicture}
    node[name=w, rectangle, fill=darkgray, minimum width=3mm, minimum height = 6cm]{};
    path[name path=wnw] (w.north west)--++(-6, 0);
    path[name path=wv] (w.west)--++(128:6);
    coordinate (B) at (intersection of wnw and wv);
    draw (w.west)--(B);
end{tikzpicture}
%
end{document}