A, B, and C are the coefficients. They are not the terms as you have written now.
A quadratic equation is in the form Ax^2 + Bx + C. Therefore, A = 1, B = 8, and C = -6.
The axis of symmetry is found using the equation -b/2a, as you've done. Therefore, the axis of symmetry is -8/2(1) = -4.
Hope this helps!! :)