Alright, so 3f-g=4 and f+2g=5.
3f-g=4
f+2g=5
Multiplying the first equation by 2 and adding it to the second, we get 7f=13 and by dividing both sides by 7 we get f=13/7. Since f+2g=5, then we can plug 13/7 in for f to get 13/7+2g=5. Next, we subtract 13/7 from both sides to get 2g=3+1/7=22/7 (since 3*7=21 and 21+1=22). DIviding both sides by 2, we get 22/14=g. Plugging that into f/39g, we get (13/7)/(22*39/14)
= (13/7)/(858/14)
= (13/7)*(14/858)
=182/6006
= 91/3003 (by dividing both numbers by 2)
= 13/429 (by dividing both numbers by 7)
= 1/33 (by dividing both numbers by 13)