To draw a straight-line at right-angles to a given straight-line from a given point on it. Let $AB$ be the given straight-line, and $C$ the given point on it. So it is required to draw a straight-line from the point $C$ at right-angles to the straight-line $AB$.  <prf> Thus, the straight-line $CF$ has been drawn at right-angles to the given straight-line $AB$ from the given point $C$ on it.