To evaluate the expression \( 3b - ac \) with the given values \( a = -2 \), \( b = 3 \), and \( c = 0.5 \), we first substitute the values into the expression.
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Substitute \( b = 3 \): \[ 3b = 3 \times 3 = 9 \]
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Substitute \( a = -2 \) and \( c = 0.5 \): \[ ac = -2 \times 0.5 = -1 \]
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Now substitute these calculated values back into the expression: \[ 3b - ac = 9 - (-1) \]
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Simplify the expression: \[ 9 - (-1) = 9 + 1 = 10 \]
Thus, the value of the expression \( 3b - ac \) is \( \boxed{10} \).