For a function $f: \mathbb{R} \to \mathbb{R}$, it is known that $f(2017) > 0$ and $f(x^2 + yf(z)) = xf(x) + zf(y)$ for all $x, y, z \in \mathbb{R}$. What is the value of $f(-42)$?

Let's think step by step and output the final answer within \boxed{}.
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