We need
#a^2-b^2=(a+b)(a-b)#
Let's factorise the expression
#x^4-13x^2+36= (x^2-4)(x^2-9)#
#=(x+2)(x-2)(x+3)(x-3)#
Let #f(x)=(x+2)(x-2)(x+3)(x-3)#
We can now do the sign chart
#color(white)(aaaa)##x##color(white)(aaaa)##-oo##color(white)(aaaa)##-3##color(white)(aaaa)##-2##color(white)(aaaa)##2##color(white)(aaaa)##3##color(white)(aaaa)##+oo#
#color(white)(aaaa)##x+3##color(white)(aaaaa)##-##color(white)(aaaa)##+##color(white)(aaaa)##+##color(white)(aaaa)##+##color(white)(aaa)##+#
#color(white)(aaaa)##x+2##color(white)(aaaaa)##-##color(white)(aaaa)##-##color(white)(aaaa)##+##color(white)(aaaa)##+##color(white)(aaa)##+#
#color(white)(aaaa)##x-2##color(white)(aaaaa)##-##color(white)(aaaa)##-##color(white)(aaaa)##-##color(white)(aaaa)##+##color(white)(aaa)##+#
#color(white)(aaaa)##x-3##color(white)(aaaaa)##-##color(white)(aaaa)##-##color(white)(aaaa)##-##color(white)(aaaa)##-##color(white)(aaa)##+#
#color(white)(aaaa)##f(x)##color(white)(aaaaaa)##+##color(white)(aaaa)##-##color(white)(aaaa)##+##color(white)(aaaa)##-##color(white)(aaa)##+#
Therefore,
#f(x>=0)#, when #x in ] -oo,-3 ] uu [ -2,2 ]uu [3, oo[ #