Let's consider a regular hexagon with unit side length. Draw a line segment of length $\sqrt{7}$ using nothing except a straightedge (that is, an unmarked ruler). The position of the segment may be chosen as you wish.
My thoughts: I've drawn all sorts of lines and tried to get the segment in one of the inside triangles, but to no avail.

The rectangle that has as basis a diagonal of the hexagon and as height the orthogonal median ( joining mean points of opposite sides) has diagonal = $\sqrt 7$.
Added after Blue comment.
All lines can be drown with an unmarked straightedge.