This bone is called a scapula, or more commonly, a shoulder blade.
The triangle shaped bone in your upper back is called the shoulder blade. This is learnt in health class.
Scapula
The triangular bone in your upper back is called the SCAPULA, more commonly known as the shoulder blade, which connects the humerus (upper arm bone) to the clavicle (collar bone).
The bone that is located between the upper and lower "leg bone" (the patella) is triangular shaped, not flat (the point of the triangle faces back), and is more square shaped when viewed head on than round. It's function is to help to keep the quadriceps tendon in line, not to protect the joint. If this is the homework question that it appears to be then I would have serious doubts about the person who asked it.
I'm saying the pelvis.
The bones of the mid and upper back are the thoracic vertebrae. The neckbones are the cervical vertebrate and the lower back bones are the lumbar vertebrae.
The scapula is either of two flat, triangular bones, each forming the back part of a shoulder in humans; shoulder blade
The upper knee. The patella is a small upside down triangle shape bone. It sits inside the tendon of the thigh muscles at the front of the knee resting in the patellar groove of the femur (thigh bone).
The scapula is the shoulder blade. This is located on the upper part of the back.
Sacrum
sphenoid bone
The long bone in the upper arm is called the humerus.