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)
