Question

Factor over the complex numbers: 36x2 + 9.

Answer 1

$36x^2+9=(6x)^2-(-9)=(6x)^2-(-3^2)\\\\=(6x)^2-(3i)^2=(6x-3i)(6x+3i)\\\\\text{Used:}\\a^2-b^2=(a-b)(a+b)\\\\i^2=-1$