c: number of carnations
r: number of roses
Rebecca's expression is
r = c + 4
Replacing with the values of the table:
c | r
0 | 0+4 = 4
1 | 1+ 4 = 5
2 | 2+4 = 6
3 | 3+4 = 7
4 | 4+4 = 8
Then, Rebecca is correct.
Isolating c from Rebecca's expression:
c = r - 4
which is Austin's expression. Then, Austin is correct too.