To find out if the triangle is a right triangle, we must check if the following equation is true:

on the left side, we have:

and on the right side, we have:

since on both sides we get 625, we have that the brace forms a right triangle.
Answer:
375 fishes
Step-by-step explanation:
12000 divided by 2 is 6000 1st year
6000 divided by 2 is 3000 2nd year
3000 divided by 2 is 1500 3rd year
1500 divided by 2 is 750 4th year
750 divided by 2 is 375 5th year