I'm assuming you want to simplify the expression since it's not equal to anything.. you can't solve for "a" unless it's equal to something.
factor
8a³ - 2a²+ 12a - 3
2a²(4a - 1) + 3(4a -1)
(4a - 1) is a like term that can be combined further.
(2a² + 3)(4a - 1)