Answer :
so what you do is evaluate g(2) then plug that value in for x in f(x)
g(2)=2^2-6(2)-7
g(2)=4-12-7
g(2)=-15
now
f(-15)=-15+8
f(-15)=-7
f(g(2)=-7
g(2)=2^2-6(2)-7
g(2)=4-12-7
g(2)=-15
now
f(-15)=-15+8
f(-15)=-7
f(g(2)=-7