If points f and g are contained in a plane, then fg is entirely contained in that plane. true or false (there is a line above fg with 2 arrows at both ends)

This is true. 
If the end point of the line FG line in the same plane, then all of the points on the line FG will lie on the same plane. But this is only true if the FG line is a straight line.
If FG is a curved line this statement can but does not have to be true. 

If gravity is one third on mars then weight also one third on mars