Let the first term and common difference = a & d respectively. \(T_{n} = \text{nth term} = a + (n - 1) d\) (A.P) Given: \(T_{4} = -6 \implies a + 3d = -6 ... (i)\) \(T_{8} + T_{9} = 72\) \(\implies a + 7d + a + 8d = 72 \implies 2a + 15d = 72 ... (ii)\) From (i), \(a = -6 - 3d\) \(\therefore\) (ii) becomes \(2(-6 - 3d) + 15d = 72\) \(-12 - 6d + 15d = 72 \implies 9d = 72 + 12 = 84\) \(d = \frac{84}{9} = 9\frac{1}{3}\)