Respuesta :

1. 18 feet below sea level because he was 15 feet then he rose 9 feet then went down 12 feet so 15 - 9 = 6 then 6 + 12 = 18

2. 23 + 5 = 28 and 28 - 7 = 21 so 21

 3.  67 cause  50 - 40 = 10 and 10 + 87 = 97 and  97 - 30 = 67