it is easier to do these without fractions...to get rid of the fractions, multiply the equation by the common denominator.
1/3x + 3/4y = 2.....multiply entire equation by 12
4x + 9y = 24
4/9x - 1/4y = 3...multiply entire equation by 36
16x - 9y = 108
now we have :
4x + 9y = 24
16x - 9y = 108
------------------add
20x = 132
x = 132/20
x = 33/5
4x + 9y = 24
4(33/5) + 9y = 24
132/5 + 9y = 24
9y = 24 - 132/5
9y = 120/5 - 132/5
9y = - 12/5
y = -12/5 * 1/9
y = -12/45 = - 4/15
so ur solution is : (33/5, -4/15)