To solve the equation \( 3(2f + 5) = 6 - 4f - 21 \), follow these steps:
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Start by simplifying both sides of the equation.
Left-hand side: \[ 3(2f + 5) = 6f + 15 \]
Right-hand side: \[ 6 - 4f - 21 = -4f - 15 \]
So the equation now looks like: \[ 6f + 15 = -4f - 15 \]
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Next, combine like terms. Add \( 4f \) to both sides: \[ 6f + 4f + 15 = -15 \] Which simplifies to: \[ 10f + 15 = -15 \]
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Now, subtract 15 from both sides: \[ 10f = -15 - 15 \] Simplifying gives: \[ 10f = -30 \]
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Finally, divide both sides by 10: \[ f = -3 \]
So the solution is: \[ \boxed{-3} \]