What is the expression x2-4 for a=b=3x=5 and y=4

The  expression [ x² - 4 ] doesn't depend on 'a', 'b', or 'y'.
It only depends on 'x'.

You have said that  3x = 5,  so x = 5/3.
Therefore, I may now write '5/3' in place of 'x' in the expression:

                               x²  -  4  =

                             (5/3)² - 4  =

                            (25/9) -  4  =

                            (25/9) - (36/9) =

                           (25 - 36) / 9  =  -11/9  =  - (1 and 2/9)