That is the harmonic balancer. The flex plate is in front of the torque convertor on the back of the motor. There is a plastic cover over it...two 10mm bolts and the cover is off. The torque convertor is quite close to the flex plate so if you want to use an extension you'll have to support it underneath with a jack or what I use, a 2 X 4 cut to just the right length. The 2 X 4 goes from the ground up to the extension with the tip of the extension in one of the holes in the flex plate. Make sure the car is on jack-stands and the wheels are chocked safety break on.
The other way to remove the balancer bolt is a large pneumatic impact wrench. This is what most people do.
