The density of a certain type of paint is 1.2 grams per cubic centimeter. a garage door that is 3 meters tall and 7 meters wide is to be painted with a 1-millimeter-thick coat of paint (on the outside only). what is the mass (in grams) of the paint that will be needed to paint the door?

We know that
density=mass/volume
solve for mass
mass=density*volume

step 1
find the area of the garage door in cm²
Area=300*700> 210000 cm²

step 2
find the volume of the paint
Volume=area*thick
thick=1 mm> 0.1 cm
Volume=210000*0.1> 21000 cm³

step 3
find the mass
mass=density*volume
density=1.2 g/cm³
volume=21000 cm³
mass=1.2*21000> 25200 grams

the answer is
25200 grams