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
