x^2-7
=(x+√7)(x-√7)
3x^2-4
=(√3x+2)(√3x-2)
x^2-2√2x+2
=x^2-2√2x+(√2)^2
=(x-√2)^2
x^4-36
=(x^2+6)(x^2-6)
=(x^2+6)(x+√6)(x-√6)
-x^4+225
=225-x^4
=(15+x^2)(15-x^2)
=(15+x^2)(√15+x)(√15-x)
x^4-6x^2+9
=(x^2-3)^2
=[(x+√3)(x-√3)]^2
=(x+√3)^2(x-√3)^2