To solve the equation \( 6u - 13u - 18 = -81 \), we first combine the like terms on the left side.
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Combine \( 6u \) and \( -13u \): \[ 6u - 13u = -7u \] So, the equation becomes: \[ -7u - 18 = -81 \]
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Next, we can isolate the term involving \( u \) by adding 18 to both sides of the equation: \[ -7u - 18 + 18 = -81 + 18 \] Simplifying both sides gives: \[ -7u = -63 \]
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Now, divide both sides by \(-7\) to solve for \( u \): \[ u = \frac{-63}{-7} \] Simplifying that gives: \[ u = 9 \]
Thus, the solution is: \[ \boxed{9} \]