The angle between 0 and 2π in radians that is coterminal with the angle
19/4π in radians is

(19/4)π =  4.75π

So we have to keep subtract 2π

4.75π - 2π = 2.75π

This is still more than 2π, so we subtract 2π again.

2.75π - 2π = 0.75π

So the coterminal angle = 0.75π = (3/4)π  radians.