Question 72201
16x^4-81 = (2x)^4 - (3)^4  or  (4x^2)^2 - (9)^2
                             =  (4x^2+9)(4x^2-9)
                             =  (4x^2+9)((2x)^2-(3)^2)
                             =  (4x^2+9)(2x+3)(2x-3)