Let the a be first term and d be common difference of an AP
Given,
4th term = - 15
a + 3d = - 15 [ using an = a + (n - 1)d]
a = - 3d - 15 [ eqn 1]
9th term = - 30
a + 8d = - 30
- 3d - 15 + 8d = - 30 [ using 1]
5d = - 15
d = - 3
putting this value in eqn 1 we get
a = - 3 (- 3) - 15
= 9 - 15 = - 6
Also we know,