Your answer is x = 1/3 or x = -1
To solve I would start by expanding the bracket and then collecting like-terms, and then re-factoring:
(3x - 1)² + 12x - 4 = 0
9x² - 6x + 1 + 12x - 4 = 0
9x² + 6x - 3 = 0
3(3x² + 2x - 1) = 0
From here you can factorise the bracket even further:
3(3x - 1)(x + 1) = 0
Then divide by 3:
(3x - 1)(x + 1) = 0
3x - 1 = 0
x = 1/3
x + 1 = 0
x = -1
I hope this helps! Let me know if you have any questions :)