translations of the graphs of sine and cosine functions

I have a test tmrw and just went over the information today so I am a little confused. Some of the graphs I get right but some of them I am getting the wrong X values.

So if i have y=-4 sin 2(x-pi/2)

so the Amplitude is 4

and the period is pi

but how in the world do i get the x axis?

I've tried everything and i'll find one way works for one type but doesn't work for another. Is it really that way? I was told to multiply pi/2 by 1/4 2/4 and 3/4 but it doesnt come out correct. Does anyone know what to add or multiply by to get the x axis? Thanks