A sloping segment between parallel lines is longer than their perpendicular separation, so its length does not give the required gap.

Draw a perpendicular to \(l\), measure \(3\text{ cm}\) along it, then draw a line through that new point perpendicular to the measuring line. This new line is parallel to \(l\) with the correct gap.

Answer

The sloping measurement is not the perpendicular distance. Measure \(3\text{ cm}\) along a perpendicular to \(l\).