Question 245988
(2x^2+9)^2=(2x^2+9)(2x^2+9)
use FOIL
first 
2x^2*2x^2=4x^4 no,problem here
outer
2x^2*9
inner
9*2x^2
last 
9*9=81 no problem here
add 2x^2*9and 9*2x^2
18x^2+18x^2=36x^2
so we have 4x^4+36x^2+81 not 36x


{2x^2+6x+9}^2 would be 4x^4+24x^3+72x^2+108x+81
neither work out to be 4x^4+36x+81

I suspect that you copied the original sqrt incorrectly ie sqrt(4x^4+36x+81)