Answer:
If your solving for x then its x=3 because you combine liked terms then divide both sides by 3.
Ok you must do the inverse of division which is multiply
Here is your answer

Since the door will be in shape of a rectangle where,
length= 80 inches
breadth= 72 inches
So,
widht of largest mattress that can fit diagonally is diagonal of the door.
i.e.





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