Winter ate one slice of pie. fall ate two slices. if there were initially 12 slices and all the slices are the same size, what fraction of the pie is remaining?

Hi there,
let a be the remaining slices
so that a+1+2=12
1: slice eaten by Winter
2: slice eaten by Fall
both ate 3 slices
and we have a total of 12 slices 
so the result should be 12-3=9 slices remaining
with algebra with have a+3=12
subtract 3 from both sides
a+3-3=12-3
a=9
Hence 9 slices left