Mary is x years old
when we write the expression for her age which represents 12 years from now, it means we have to add 12 years in the age of Mary, and she is x years old now
so the expression will be:
x+12
and the expression which represents her age two years ago from now when she is x years old is:
x-2
if we know the x, we can calculate easily her age for above two expressions.