Answer: perpendicular
Step-by-step explanation:
One of the properties of two parallel lines in a plane is:
In this case, the parallel lines are the sidelines and the transversal is the 50 yard line. Since the angle between one of the sidelines and the 50 yard line is 90 degrees (perpendicular), then the angle between the other sideline and the 50 yard line has to be congruent, so it's also perpendicular.