Let a line \( L: 2x + y = k,\ k > 0 \) be a tangent to the hyperbola \( x^2 - y^2 = 3 \). If \( L \) is also a tangent to the parabola \( y^2 = \alpha x \), then \( \alpha \) is equal to:
\( 24 \)
\( -24 \)
\( 12 \)
\( -12 \)