If the system of linear equations:\( x + y + 2z = 6 \),\( 2x + 3y + az = a + 1 \),\( -x - 3y + bz = 2b \),where \( a, b \in \mathbb{R} \), has infinitely many solutions, then \( 7a + 3b \) is equal to:
\( 16 \)
\( 22 \)
\( 12 \)
\( 9 \)