Steel Design I am having trouble with the concepts of elastic/plastic neutral axes. What determines which axis you take the modulus about?

Steel Design

I am having trouble with the concepts of elastic/plastic neutral axes. What determines which axis you take the modulus about? How do you find the plastic neutral axis of a non-symmetrical shape?

Determine the elastic neutral axis, elastic section modulus, plastic neutral axis, and plastic section modulus for a WT15x45 modeled as two rectangles forming the flange and the stem.