how do we know whens our exam because theres different exam times and i don't know which one do i belong in?

Which course are you referring to? The exam times for each course should be the same

i meant how do i find my room, is it base on alpha split if it is how do i use it? my last name is lau

Yes it's based on last name. Find the section where "lau" will fit, for instance if there is an Alpha Split from J-M, then you belong in that section.

most likely all sections/last names for each couirse are in the same room