If a, start subscript, 1, end subscript, equals, 6a
1
​
=6 and a, start subscript, n, end subscript, equals, minus, 3, a, start subscript, n, minus, 1, end subscript, minus, na
n
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=−3a
n−1
​
−n then find the value of a, start subscript, 3, end subscripta
3
​
.