The damaged U-joint is clearly visible.

There are four bolts that connect the flange to the yoke. The bolt heads require a 12mm 12point socket. A 6 point socket will not fit, and a box end wrench will not generate enough torque.

Use a 12mm 12 point socket and a 16 inch breaker bar will generate enough torque to remove the bolt. Here a 1/2" drive socket set is used. 3/8" socket set may work as well, but the larger drive set will tolerate a bit more torquing.

Loosen all four bolts in a criss-cross pattern.