Auniform, thin, solid door has height 2.30 m, width 0.875 m, and mass 24.0 kg. (a) find its moment of inertia for rotation on its hinges.

A uniform thin solid door has height 2.20 m, width .870 m, and mass 23.0 kg. Find its moment of inertia for rotation on its hinges. Is any piece of data unnecessary? So far, I don't understand how to calculate moments of inertia for things like this at all. I can do a system of particles, but when it comes to any ridgid objects, such as this door or rods or cylinders, I don't get it. So basically I have no idea where to even start with this. 

so A

Answertechnician (a) is correct;