g(f(-3)) means you are plugging in for g(x) whatever value you get for f(-3).
f(x) = 3x^2+6x-11
f(-3) = 3(-3)^2 + 6(-3) + 11
f(-3) = 27 -18 +11
f(-3) = 20
g(f(-3)) becomes g(20)
Now plug 20 in for x in g(x) = -2x + 9
g(20) = -2(20) + 9
g(20) = -40 + 9
g(20) =-31
g(f(-3)) = -31
