First, it is good to note that this is a kite, and the line of symmetry will be the horizontal line y=6.

Then, split the kite into two triangles, and you can find the area of the two triangles and sum them together.

Then, to find the length of AB, use Pythagoras theorem to solve it. Lastly, to find equation of line, you need to solve for the gradient, and the y-intercept.

Hope this helps! Tell me if you need any more help.