Explanation

Cross-multiplying 6b(2a − b) = a(6a − b) gives 12ab − 6b² = 6a² − ab, i.e. 6a² − 13ab + 6b² = 0. This factorises as (3a − 2b)(2a − 3b) = 0. If 3a = 2b then (a+b)/(a−b) = 5k/(−k) = −5; if 2a = 3b then it equals 5k/k = 5. Hence the ratio is −5 or 5, option (d).