To draw a straight-line perpendicular to a given infinite straight-line from a given point which is not on it. Let $AB$ be the given infinite straight-line  and $C$ the given point, which is not on ($AB$).  So it is required to draw a  straight-line  perpendicular to the given infinite straight-line $AB$ from the given point $C$, which is not on ($AB$). <prf>   Thus, the (straight-line) $CH$ has been drawn perpendicular to the given infinite straight-line $AB$ from the given point $C$, which is not on  ($AB$).