How to find coefficients $a$ and $b$ when given the curl?
I'm guessing the parameters are supposed to be dimension indices, and that you're supposed to replace them with either $1$, $2$, or $3$ (or whatever notation you use to denote distinct dimensions in 3-D). $f_a$ should be the $a$-component of the vector field in question, and $\frac{\partial}{\partial x_b}$ should be the partial derivative in the $b$th direction.
The answer should follow directly from the definition of the curl.