[tex]f(x)=x^2-8\\\\g(x)=2x-5\\\\f(g(x))\to\text{instead of x write the equation of the function g(x)}\\\\f(g(x))=(2x-5)^2-8=(2x)^2-2(2x)(5)+5^2-8\\\\=4x^2-20x+25-8=4x^2-20x+17\\\\\text{used}\ (a-b)^2=a^2-2ab+b^2[/tex]
[tex]g(f(x))\to\text{instead of x write the equation of the function f(x)}\\\\g(f(x))=2(x^2-8)-5=(2)(x^2)+(2)(-8)-5=2x^2-16-5=2x^2-21[/tex]
