Ok so you've tried to use Biot-Savart's law but you're stuck right after this.

I had a little time so I did part a).

Latex is down so I redirect you to PHF sister's site. Go at

From PHF - Math Help Forum.

Can you draw the vectors dl and r on the figure? If so, what is the angle between them? What does that tell you about the value of the cross product?

If you answer these questions, you're almost finished. Hopefully you can take it from there. If you have any problem, feel free to ask.