Can the Moment of Inertia about x-x axis be negative?
No, the Moment of Inertia about x-x axis, measured in Second Moment of Area cannot be negative.
What unit is used to measure Moment of Inertia about x-x axis?
Moment of Inertia about x-x axis is usually measured using the Millimeter⁴[mm⁴] for Second Moment of Area. Meter⁴[mm⁴], Centimeter⁴[mm⁴] are the few other units in which Moment of Inertia about x-x axis can be measured.