The cost (in dollars) of making n feet of cabinet is represented by C = 18n + 45
The equation you have represents the cost for n feet of cabinet. If you know what the cost is C, you can solve for n.
The cost is given as $441. Plug into equation.
441 = 18n + 45
Subtract 45 from both sides
396 = 18n
Divide by 18 to get n
22 = n
The answer is 22 feet of cabinet.