y= 2(4)^x should be your answer.
Answer:

Step-by-step explanation:
The fish population after t years can be modeled by the following equation:

In which F(0) is the initial population and r is the constant rate of decay.
Year one the fish population was 18000
This means that 
In year three the fish population was 8000 fish.
Two years later, so 






So

I don't really know if the question is complete but...
y = x -10 can be a solution
so y would be -7 when x is 3
186cm
You add all the sides up, so 42+42+51+51 which equals 186cm