For the Hacking the Hacker gig, you will need to get into the 6th Street base in the Megabuilding and plant a virus. While there, you will also need to take out their hacker, if you can.
There is a pretty easy route to take to do this without being seen by anyone, but it is not very obvious when you first arrive. Head up in the elevator and then crouch as soon as the doors open.
Head to the right and you will see a small shutter window just before two enemies. Sneak up to the window and you will be able to open it and jump inside, avoiding the first two guards.
Loot anything in the room that interests you, then head out the door on the far side. Make sure you stay crouched so nobody sees you. Make your way to the left, along the wall, and you will come to a broken piece of the pathway. You can jump across, but make sure you hack the camera first.
After you hack the camera, jump the gap then make your way behind the grating on the far side. There is one route to take here that will lead to a room with the hacker and guard in it. The guard is sitting at a table, and if you wait a short time the hacker will move to a server stack in the back of the room. Sneak in and take out the hacker, then go and deal with the guard. You can execute both of them silently by sneaking up on them.
Once both of them are dealt with, you can upload the virus onto the console in the center of the room. Now, you need to sneak back out again so just retrace your steps, making sure to sneak past the first two guards after you climb back out the window. When you get outside, the drop off for your reward is the selling point just at the front of the building.