Let a be the first term
Let d be the common difference
a + d = 7
S4 = 4/2 (2a +3d) = 12
Simultaneous equation:
a+d =7 // x 6 4a +6d = 12
Difference btween these two
6a + 6d = 42
4a +6d = 12
2a = 30
a = 15
d = 7 -a
thus d = -8, a = 15
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