To solve these problems, substitute into the function $f(x) = x^2 - 2x - 8$.

C) Evaluate $f(3x)$:

1. Substitute $3x$ for $x$ in the function: $f(3x) = (3x)^2 - 2(3x) - 8$
2. Simplify: $f(3x) = 9x^2 - 6x - 8$

D) Evaluate $f(x+4)$:

1. Substitute $x+4$ for $x$ in the function: $f(x+4) = (x+4)^2 - 2(x+4) - 8$
2. Expand and simplify: $f(x+4) = (x^2 + 8x + 16) - (2x + 8) - 8$ $f(x+4) = x^2 + 6x$

E) Evaluate $f(-2x+1)$:

1. Substitute $-2x+1$ for $x$ in the function: $f(-2x+1) = (-2x+1)^2 - 2(-2x+1) - 8$
2. Expand and simplify: $f(-2x+1) = (4x^2 - 4x + 1) + (4x - 2) - 8$ $f(-2x+1) = 4x^2 - 9$

These are the simplified expressions for each evaluation.