The praying figures with outstretched hands like those depicted in ceiling painting in the catacomb of saints peter and marcellinus are called:

Orans (or orante in the Latin original).

A believer who prays with outstretched hands is a very ancient religious representation. Such figures were found in pre-Christian cultures, such as Sumerian (3.000 BC). Jews also practiced this kind of worshipping, and early Christians probably adopted it from them, as it resembled of crucified Christ.

2.25 because you subtract the two