If f(x)=x^4+8, g(x)=x-8 and h(x)=sqrt (x), then

plug it in
f(g(h(x)))=

f(g(h(x))) = f(h(x) - 8)

f(h(x) - 8) = h(x)^4 + 8 = (sqrt(x))^4 + 8 = x^2 + 8