If the third proportional of $ 3x^2 $ and 4xy is 48, then the positive value of y is :

To find the third proportional:

$3x^2 : 4xy = 4xy : 48$

So,

$3x^2 \cdot 48 = (4xy)^2$

$144x^2 = 16x^2 y^2$

Divide both sides by $16x^2$:

$9 = y^2$

So the positive value:

$y = 3$