To ease the understanding, let's write it in degrees and not in Radian:
Formula of conversion: Degree°/180 = Radian/π
Convert 7π/4 in degrees: = (7x180)/4 = 315°
This angle is in the IV th Quadrant of the UNIT circle. So you can write 315° or - 45° .
sin(7π/4) = sin(315°) = sin( - 45°) = - sin(45°) = - (√2)/2