flatband and threshold voltages

An Al-silicon dioxide-silicon MOS capacitor has an oxide thickness of 450Angstroms and a doping of Na=10^15 cm^-3.  The oxide charge density is Q'ss=3x10^11cm^-2.  Calculate (a) the flat-band voltage and (b) the threshold voltage.  Sketch the electric field through the structure at the onset of inversion.