Please explain how to get the answer, with step by step explination. I would like to know, so I can do it by my self in the future.

Answer:
(-8, 135°)
Step-by-step explanation:
(8, -45°) has magnitude 8 and direction -45°.
If we add 180° to the direction and change the sign of the magnitude, we'll have the same point.
(-8, -45° + 180°) = (-8, 135°)