Question 75945
Are you supposed to be factoring or solving for x?
16x^2-36=?
This is the difference of perfect squares the rule for that is {{{highlight(a^2-b^2=(a+b)(a-b))}}}.

{{{(4x)^2-(6)^2=(4x+6)(4x-6)}}}
:
9x^2+81=?
This is the sum of perfect squares and is prime.  It can't be factored or solved unless you have started using the complex number system.
:
If you have started using complex numbers or need these solved for x let me know.
Happy Calculating!!!