Two concentric conducting spherical shells of radii $a$ and $b$ ($b > a$) carry charges $+Q$ and $-Q$ respectively. The electric field at a distance $r$ from the center, where $a < r < b$, is:

A solid metallic sphere has a charge $+ 3{\cal Q}.$ Concentric with this sphere is a conducting spherical shell having charge $- {\cal Q}.$ The radius of the sphere is a and that of the spherical shell is $b(b > a).$ What is the electric field at a distance $R(a < R < b)$ from the centre?