A tangent and a normal are drawn at the point \( P(2,\,-4) \) on the parabola \( y^2 = 8x \), which meet the directrix of the parabola at the points \( A \) and \( B \) respectively. If \( Q(a,\, b) \) is a point such that \( AQBP \) is square, then \( 2a + b \) is equal to:
