What is the common factor of 4$x^2$+9+12x and 4$x^2$-9 ?

4$x^2$+9+12x = $(2x{)}^2$+ $3^2$+ 2(2x)(3) = $(2x+3{)}^2$

$4x^2$-9 = (2x+3)(2x-3)

So, the common factor is 2x+3.