4x² - (2x + 3)²
4x² - (2x + 3)(2x + 3)
4x² - (2x(2x + 3) + 3(2x + 3))
4x² - (2x(2x) + 2x(3) + 3(2x) + 3(3))
4x² - (4x² + 6x + 6x + 9)
4x² - (4x² + 12x + 9)
4x² - 4x² - 12x - 9
-12x - 9
-3(4x) - 3(3)
-3(4x + 3)
The square root of a -36 simplifies to "6i". So you have 3+/- [6i/6]. The 6's cancel each other out leaving 3+/-i. So the "a" is 3 and the "b" is 1
