Which equation is y is equal to 9 x squared plus 9 x minus 1 rewritten in vertex form?

Factor 9 from the terms containing x: y = 9(x² + x) − 1. Complete the square using (x + 1/2)² = x² + x + 1/4, so y = 9[(x + 1/2)² − 1/4] − 1 = 9(x + 1/2)² − 13/4. Therefore, the vertex form is choice C.

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