Could you just do this using graphical method? If they are saying "express arcsinx in terms of y" this implies we need to do something to y=arccosx such that its the same as arcsinx. So if you do -arccosx +pi/2 you see that after this transformation the arccosx graph is exactly the same as arcsinx graph, therefore arcsinx=-y+pi/2, so for the next part we just combine all of this and do y+-y+pi/2 which is just pi/2