If f(x) = x^3 + x^2 - 1, then
f(-x) = (-x)^3 + (-x)^2 - 1
f(-x) = (-1)^3 * x^3 + (-1)^2 * x^2 - 1
f(-x) = (-1) * x^3 + 1 * x^2 - 1
f(-x) = -x^3 + x^2 - 1
So,
g(x) = f(-x) - 5
g(x) = -x^3 + x^2 - 1 - 5
g(x) = -x^3 + x^2 - 6 <----- this is the answer
I hope this helps. =)