Fix the mine's air purifier

Talk to the super mutant inside the house outside the Broken Hills mine's entrance. He will tell you the mine needs parts for the mine purifier and asks you to go to New Reno to pick it up. You can either take the reward for money, or, you can decline, and recieve a combat shotgun when you finish the quest.

Talk to Renesco the Rocketman in New Reno.

Return to the super mutant and he will ask you to enter the mine to fix the purifier. The mine is filled with poison gas so make sure you have gas protection or equip a power armor when you do this quest.

Return to the super mutant and he will give you a combat shotgun (or $500 if you asked for a reward earlier).