 Solution

The surface is obtained by revolving the graph $z=f(x)$ about the $x$-axis. It has a central cylindrical section of radius $b$, two smooth transition collars, and unit spherical caps centred at $(\pm 3,0,0)$; its projection onto the $(x,z)$-plane is the region $|z|\le f(x)$.

For a surface of revolution, the Gaussian curvature of a surface of revolution isThus the cylindrical region has $K=0$. On each transition collar, $K<0$ where $f^{\prime \prime }>0$, vanishes at the unique inflection circle, and is positive where $f^{\prime \prime }<0$. The spherical caps have $K>0$.