I'll think this through step by step. When you place a coin in a cup and slam it upside-down on a glass table, the coin will be between the table surface and the cup. When you look at the reflection in the mirror beneath the table, you're seeing the bottom side of the coin through the glass. Since the mirror shows heads, you're seeing the heads side of the coin reflected in the mirror. In a mirror, the image is reversed left-to-right, but the "heads" and "tails" sides of a coin would not be flipped in this reflection - you'd still recognize heads as heads. If you take away both the cup and the mirror, you'll see the opposite side of the coin compared to what you saw in the mirror. Since the mirror showed heads, you'll see tails when looking directly at the coin from above the table.