To simplify the expression \( (3x^2 + \frac{3}{4}x - 1) - (x^2 - \frac{1}{4}x) \), we first distribute the negative sign across the second set of parentheses:
\[ 3x^2 + \frac{3}{4}x - 1 - x^2 + \frac{1}{4}x \]
Next, we combine like terms.
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For the \( x^2 \) terms: \[ 3x^2 - x^2 = 2x^2 \]
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For the \( x \) terms: \[ \frac{3}{4}x + \frac{1}{4}x = \frac{3}{4}x + \frac{1}{4}x = \frac{4}{4}x = x \]
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The constant term remains the same: \[ -1 \]
Putting it all together, we have: \[ 2x^2 + x - 1 \]
Thus, the simplified expression is \( 2x^2 + x - 1 \).
The correct answer is C. \( 2x^2 + x - 1 \).