How to construct a midpoint and how to define a perpendicular bisector.
How to determine whether two lines in space are parallel or perpendicular.
How to find a vector normal (perpendicular) to a plane given an equation for the plane.
How to find the angle between planes, and how to determine if two planes are parallel or perpendicular.
How to define the components of a vector.
How to find the resultant of two forces when they are parallel, opposite, or perpendicular in direction.