hi, sorry for the late reply.
Anyways!
We know that AC is the base.
The formula for area of ABC is, lh/2, so l*h = 480.
We also know that AC is the l.
Can you tell the height now?
Then, use the height, and using pythog, calculate the sides.
(only one is required as it's Isoceles, and remember that the base needs to be divided by two to do the pythog!)