To construct a rectilinear angle equal to a given rectilinear angle at a (given) point on a given straight-line.Let $AB$ be the given straight-line,  $A$ the (given) point on it, and  $DCE$ the given rectilinear angle.So it is required to construct a rectilinear angle equal to the given rectilinear angle $DCE$ at the (given) point $A$ on the given straight-line $AB$.  <prf> Thus, the rectilinear angle $FAG$,  equal to the  given rectilinear angle $DCE$, has been constructed at the (given) point $A$ on the given straight-line $AB$.