If $\mbox{curl }\vec F=(x^2+z^2)\vec j +3\vec k$, find $\int_C\vec F\cdot d\vec r$, where $C$ is a circle of radius 3, centered at the origin, with

(a) $C$ in the $xy$-plane, oriented counterclockwise when viewed from above.
$\int_C\vec F\cdot d\vec r =$

(b) $C$ in the $xz$-plane, oriented counterclockwise when viewed from the positive $y$-axis.
$\int_C\vec F\cdot d\vec r =$