39 x 57 =

9 x 7 = 63 -> 6, (3)
3 x 7 + 9 x 5 = 66 -> 66 + 6 = 72 -> 7, (2)
3 x 5 = 15 -> 15 + 7 = (22)

()を順番に拾うと 2223