To substitute \( x = 3 \) into the equation \( 3x + y = 15 \), follow these steps:
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Replace \( x \) in the equation with 3: \[ 3(3) + y = 15 \]
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Calculate \( 3(3) \): \[ 9 + y = 15 \]
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To solve for \( y \), subtract 9 from both sides: \[ y = 15 - 9 \]
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Calculate the right side: \[ y = 6 \]
So, when \( x = 3 \), \( y = 6 \). The solution is \( (3, 6) \).