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)