Question 183237
Your question isn't clear. There are no radicals.
f(x)=16-x^2 
g(x)=4-x
f(x)-g(x)
=16-x^2-(4-x)
=16-x^2-4+x
=-x^2+x+12
=-(x^2-x-12)
=-(x-4)(x+3)
.
Ed