I would make 4 variables.
On touch begin, I would set StartX and StartY as Touch.X and Touch.Y.
On touch end, I would set EndX and EndY as Touch.X and Touch.Y once again.
What I'd have had there would be the start and end of my drag. From there I could use the angle(StartX,StartY,EndX,EndY) to find the angle the player has drawn. From there, I can launch the projectile to the Angle I just calculated. If it's inverse or anything like that, you could just use -(angle(StartX,StartY,EndX,EndY)) or something mathematical along that lines.
Hope this helps!