Creating 3d objects

Hi everyone, can you show me how to creat the object that formed by 2 surfaces:

a) z = (x^2 / a^2) + (y^2 / b^2) and y = x^2 + z^2

b) z = (x^2 / a^2 ) - ( y^2 / b^2 ) and y = z^2 - x^2.

Thank you very much. I am really appriciated.