A woman bought a printer for $30000 if it's price depreciates by
per annum, how much will she be able to sell it 3 years later.

First a more theoretical approach.

Suppose that we have a point (x, y), if you want to do a reflection over the y-axis, this point will transform into (-x, y).

This reflection leaves unchanged the distance between the point and the closest point to the axis of reflection.

Below, you can see a rough sketch of the situation.

Where the distance between the point S and the axis of reflection is 2 units, and the same for the distance between S' and the axis of reflection.

"Do you think you would see the same characteristic if you drew the line segment connecting T with the reflecting line and then T' with the reflecting line? How do you know?"

Yes, because as i wrote above, the distance between the point and the axis of reflection is invariant in this type of transformations.