Step-by-step explanation:
[tex]f(f(x)) = f( {x}^{2} + 4)[/tex]
[tex] = {( {x}^{2} + 4) }^{2} + 4[/tex]
[tex] = {x}^{4} + 4 {x}^{2} + 16 + 4[/tex]
[tex] = {x}^{4} + 8 {x}^{2} + 20[/tex]
