Answer:
It will cost a total of $27.00. 
Explanation:
Manny's bedroom door is 9 feet tall and 3 feet wide. 
The new door costs $1.00 per square foot. 
And we want to find the total cost of the new door. 
Since the door is a rectangle 9 feet tall and 3 feet wide, its total area is: 

And since it costs $1.00 per square foot, we can simply multiply it to the total area. Hence: 

It will cost a total of $27.00.