Okay, here goes. I have to admit that I did have to research the first two numbers, and double-check the third number. However, I did the actual arithmetic all by myself, although I confirmed my answer using the calculator on my computer.
(The last number of Scully's Badge No., JTT0331613
) = 3
plus (Marjorie Butters' age in the 7th season episode 15) = 118
multiplied by (The last number of the year in which a UFO crashed in Roswell, 1947
) = 7
So, we have: 3 + 118 x 7 = 121 x 7 = 847
The Magic Number is 847.