a uniform solid block has mass 0.172 kg and edge lengths a=3.5cm, b=8.4cm, and c=1.4cm (c is the height of the rectangular solid).Calculate its rotational inertia about an axis through one corner and perpendicular to the large faces.

i know the formula is integral of r^2 dm, but i have no idea what to do here. i have r^2 = a^2 + b^2. but i don't know how what to substitute for dm.