If you sketch the graph...plot the vertex and focus...you will see that the parabola opens to the left.
The standard form for this parabola is , where
Now, the vertex is at (h, k) which makes h = 8, and k = 6.
The focus is defined as
Since your focus is at (2, 6), that would make
Solve for a.
Now you have a, h, and k.
Substitute into , and there you have it.