In 6 liters of the 50% juice you have exactly 3 liters of the pure juice.


You add "x" liters of the pure juice to it.


Then you have  (3+x) liters of the pure juice in the mixture, and the total volume of the new mixture is  (6+x).


You want


     = .        ( <<<---===   = 0.75 = 75% )


Simplify and solve for x:


     4*(3+x) = 3*(6+x)

     12 + 4x = 18 + 3x

     4x - 3x = 18 - 12

     x = 6.


Answer.  You need to add 6 liters of the pure pineapple juice.


Check.   After adding, you will have 3 + 6 = 9 liters of the pure juice and the total volume of  6 + 6 = 12 liters,

         which gives you the concentration of the new blend of   = , exactly what you want / (what you need).