If you have four lines how can you position them so that three are concurrent with each other and two are parallel to each other?
Concurrent lines are lines that intersect at a single point.
Well, a figure cannot be shown here, but try to draw/imagine.
For instance, in an x-y plane, you have the three concurrent lines that intersect at the origin. Now, in order to have one pair of parallel lines, the fourth line must lie in the z-axis, and must be parallel to only one of the three lines. It cannot lie on the x-y plane also because it will intersect the other two lines (which will make them concurrent with each other - which we cannot assume).
hope it's clear ^^;