Question: User: i think we let the $3\cos 2x$ as $z$
1. The problem is to substitute $3\cos 2x$ with a new variable $z$ to simplify expressions or equations.
2. The substitution rule allows us to replace a complex expression with a simpler variable to make solving easier.
3. Here, we define:
$$z = 3\cos 2x$$
4. This means wherever you see $3\cos 2x$, you can write $z$ instead.
5. This substitution helps reduce complexity in equations involving $3\cos 2x$.
6. Remember to substitute back $3\cos 2x$ for $z$ after solving to get the solution in terms of $x$.
Final answer: $z = 3\cos 2x$