If a car needs three mirrors and four doors and you have 12 mirrors and 12 doors what is the excess reactant

First, we calculate the number of cars that can be made using the given materials.
Each car needs three mirrors, we have 12 mirrors, so:
12/3 = 4 cars may be made on the basis of mirrors
Each car needs four doors, we have 12 doors, so:
12/4 = 3 cars may be made on the basis of doors

Therefore, mirrors are the excess reactants as there are more mirrors than needed to produce three cars.