The player started the game with 0 points
then she earned 50 points , but later lost 80 points.
If we represent such on the number line, first we start at the origin (0). Then we move to the right 50 units (to the point 50), but then we are forced to move 80 units to the left. so we go back the 50 units to the zero, and 30 units more to the left (to complete suntracting the 80 units)
So the final position on the number line shows a point on the left of the oigin (zero) located 30 units to the left, and the label reading "-30".