The condition implies that a + b = k-1, (1) a - 2b = 5, (2) k = 3. (3) First step to simplify gives a + b = 2, (4) a - 2b = 5. (5) Distract (4) from (5). You will get -3b = 3, hence, b = -3. Then from (4) a = 2 - b = 2 - (-3) = 5. Answer. a = 5, b = -3, k = 3.