X^2 + x + 1 + 3x^2 + 3x + 2. Just combine the like terms:
x^2 + 3x^2 + x + 3x + 1 + 2
4x^2 + 4x + 3. <--- the sum.
Answer:
(a) 100 fishes
(b) t = 10: 483 fishes
t = 20: 999 fishes
t = 30: 1168 fishes
(c)

Step-by-step explanation:
Given


Solving (a): Fishes at t = 0
This gives:






Solving (a): Fishes at t = 10, 20, 30






Solving (c): 
In (b) above.
Notice that as t increases from 10 to 20 to 30, the values of
decreases
This implies that:

So:
The value of P(t) for large values is:




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The area of a trapezpid is 1/2h×(b1+b2)