Let's let $u = 3-x$. Then, $du = -dx$.
Now, we can rewrite the integral in terms of $u$:
$$\int \frac{dx}{3-x} = \int \frac{-du}{u}$$
Notice that the denominator has become $u$ instead of $3-x$, and the $dx$ has become $-du$.
Now, we can simplify the
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