Go to turn in the two contracts and you will be approached by Astrid. Travel to Volunruud and meet with Amaund. He will give you a few things. Return to Astrid with them and she will tell you to take the amulet to Delvin of Thieves Guild fame. You would have had to get there during the story mission "A Cornered Rat." If you need to know how to get there, check the section for that mission and find your way to the Raggad Flagon.
After meeting up with Delvin, return to Astrid to get your next target.