There are two situations: one is to cross a point on a straight line as the perpendicular of a known straight line, and the other is to cross a point outside the straight line as the perpendicular of a known straight line.

Drawing method: first, use one side of the ruler to coincide with the known straight line, then use a right-angle side of the triangle to coincide with one side of the ruler (the side coincident with the straight line), then translate the triangle along the edge of the ruler until the other right-angle side of the triangle is close to the point, and finally draw a line along the right-angle side and mark it with a right angle!