Please help with steps and explanation. Please and thank you.

The figure depicts a simplistic optical fiber: a plastic core (n1 = 1.56) is surrounded by a plastic sheath (n2 = 1.44). A light ray is incident on one end of the fiber at angle 0. The rays is to undergo total internal reflection at point A, where it encounters the core-sheath boundary. (Thus there is no loss of light through that boundary.) What is the maximum value of @ that allows total internal reflection at A? Number i Units . (degrees)