If the orthocentre of the triangle formed by the lines \( 2x + 3y - 1 = 0 \), \( x + 2y - 1 = 0 \) and \( ax + by - 1 = 0 \), is the centroid of another triangle, whose circumcentre and orthocentre respectively are \( (3, 4) \) and \( (-6, -8) \), then the value of \( |a - b| \) is _______.
