I'm assuming the ends of the duct are open (i.e., no surface covering either end).
A rectangular prism has length, width, and height.
The duct is 10 feet long, 30 inches wide, and 30 inches high.
Therefore, the total surface is the sum of four identical areas -- each one belonging to a rectangle measuring 10 feet by 30 inches.
The area for a rectangle is the product of its two dimensions (measured in identical units, of course).
Area = Length × Width
or
Area = Length × Height
Something seems wrong with your information because I don't get 105 square feet.