Why joint distribution exercises are going badly

Could someone tell me what happened? Do not understand.
I have an exercise:
$$
f (x) = 3x \
0 <y <x < 1\ Find\ P(X > frac12 | Y> frac34)
$$

I found it:
$$
P (X> frac12 | Y> frac34) = frac {P (X> frac12 cap Y> frac34)} {P (Y> frac34)}
$$

which involves:
$$
frac { int_ frac12 ^ 1 int_ frac13 ^ x3x} {[marginal PDF_y] int_ frac13 ^ 1 frac {3-y ^ 2} {2}}
$$

for the upper part I get $$ frac12 $$ and for the background $$ frac {68} {81}
\
frac12 div frac {68} {81} = frac {81} {136} $$

which is not the right answer.
Explain what you like what I did wrong. My teacher insists that I should use the marginal distribution in the denominator.