Look at the figure below:
Can you see that, in both cases, the angle between the mirror (gray line) and the arrow on the left is the same as the angle between the mirror and the arrow on the right?
You can prove this law yourself. Get a small mirror and a laser pointer (never look into the laser pointer and never point it at someone else). Have someone stand facing a wall, holding the mirror so that it points at the wall. Turn the pointer on and position it so that it shines on the mirror. The laser light will bounce off the mirror and hit the wall. Can you find it? Now move the pointer so that the light hits the mirror at a different angle. Does the spot on the wall move? You can also try keeping the laser still and moving the mirror. You should see the laser spot move whenever the angle between it and the mirror changes - it doesn't matter whether it's the pointer or the mirror that moves.