1\2x+1y=3/4, y-2x=3 , do i divide all the fractions by 4 to get rid of the fraction or by 2?

You multiply both sides of the first equation by  4.
Then you have . . .
                            -2x + 4y  =  3     and     y - 2x  = 3 .

If you want to get rid of a fraction, you multiply it by itsdenominator.
BUT ... whatever you do to one term on one side of the equation, you
have to do the same to all the terms on that side, AND then you have to
do the same thing to all the termson the OTHER side.