Answer:
10 + 4 i
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First of all youre not going to get a direct answer to all of these questions, youre going to have 3 finally equations(because thats the amount of variables you have).
Answer:
radius 12 units = 452.16 units²
diameter 16.8 units = 221.5584 units²
radius 3.4 units = 36.2984 units²
diameter 10 units = 78.5 units²
Formula for area of circle: 
6.75 is the answer to your question