Let \( (a,b) \) be the point of intersection of the curve \( x^{2} = 2y \) and the straight line \( y - 2x - 6 = 0 \) in the second quadrant. Then the integral \( I = \int_{a}^{b} \dfrac{9x^{2}}{1 + 5^{x}} \, dx \) is equal to:
\( 24 \)
\( 18 \)
\( 27 \)
\( 21 \)