a). since you've done the other problems, you surely saw some gradients and divergences. The easiest way to do this is piece by piece. First take the gradient of f to get a vector field. Then simply apply div to this vector field to get the answer.
once you get this you will see the answer for part b
I agree with ice109- if the problem asks you to find "div grad f", I would recommend that you first find the grad of f and then the div of that! I am assuming that, since you were given a problem like this, you know what "div" and "grad" are.
Related Threads for: Vector calculus. Divergent vector field.