HELP PLEASEEEEEEEEEEE

Answer:
Which one is at the bottom? if the one at the bottom is 0 then the answer is 0. If its not then its related to pemdas or something else
Step-by-step explanation:
Answer:
? = 4
4 is the answer
Step-by-step explanation:
heart = 21
rainbow = 7
unicorn horn = 9
so, we have
heart + rainbow + rainbow = 35
21 + 7 + 7 = 35
heart + unicorn horn + rainbow = 37
21 + 9 + 7 = 37
rainbow + unicorn horn + unicorn horn =25
7 + 9 + 9 = 25
unicorn horn + unicorn horn + rainbow - heart = ?
9 + 9 + 7 - 21 = 4