# Why joint distribution exercises are going badly

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