I don't see a drawing of the quadrilaterals, so I don't know what the perimeter of quadrilateral P is. But whatever the perimeter of P is, Q will be 1/3 of that. Perimeter is a length, so even though it may pertain to a 2-dimensional object, it is still a 1-dimensional, linear measure. When two objects are similar (same shape, but scaled up or down by a scale factor), all corresponding linear measures have the same scale factor.
If you were asked about area or volume, that would be a different matter. In the case of area, you would square the scale factor, and in the case of volume, you would cube the scale factor.
