Axis of symmetry is x = 1 , vertex is at ( 1 , 15 ) . Explanation: f ( x ) = − 3 x 2 + 6 x + 12 = − 3 ( x 2 − 2 x ) + 12 = − 3 ( x 2 − 2 x + 1 ) + 3 + 12 = − 3 ( x − 1 ) 2 + 15 . Comparing with standard vertex form of equation f ( x ) = a ( x − h ) 2 + k ; ( h , k ) being vertex. Here h = 1 , k = 15 . So vertex is at ( 1 , 15 ) . Axis of symmetry is x = 1