To find the x-coordinate of the vertex of the parabola given by the equation y = 3x² + 9x, we can use the formula for the vertex of a parabola in standard form, which is y = ax² + bx + c. The x-coordinate of the vertex can be calculated using the formula:
x = -b / (2a)
In our equation, a = 3 and b = 9. Plugging these values into the formula gives:
x = -9 / (2 * 3) = -9 / 6 = -3/2
Therefore, the x-coordinate of the vertex of the parabola is -3/2.