Look at the chair next to the door. Take the key.
Drag the key to the left lock on the chest.
Now you have to change the key to open the right lock. Tap the key in your inventory. Tap the ring on the key and the key will change.
Tap the key in your inventory again to go back.
Drag the key to the right lock and the chest will open.
Put in the pattern you saw on the ground:
Row 1: X O O X
Row 2: O X O O
Row 3: O X X O
Row 4: X X O O
Continue to the next Can You Escape the Dark Mansion level!