The nth term of an arithmetic sequence can be written as a + (n - 1)*d, where a is the first term and d is the common difference.
We have a2-a6+a4=-7
=> a + d - a - 5d + a + 3d = -7
=> a - d = -7
=> d = a + 7
a8 - a7 = 2*a4
=> a + 7d - a - 6d = 2*(a + 3d)
=> d = 2a + 6d
=> 2a + 5d = 0
Now substitute d = a + 7 in 2a + 5d = 0
=> 2a + 5(a + 7) = 0
=> 2a + 5a + 35 = 0
=> 7a + 35 = 0
=> a = -35/7
=> a = -5
d = a + 7 = -5 + 7 = 2
The first term is -5 and the common difference is 2
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