To find how much water was in the pool originally, you need to write the function for how fast the water is leaving the pool.
Your function is
y=-8x+b
y is amount of water in the pool
x is how many hours
b is the original amount
Since you know y and x, plug those into the function.
132=-8(6)+b
132=-48+b
b=180
The original amount of water in the pool was 180 gallons.
I didn't mean to reanswer the question, I'm sorry! I didn't see that there was already an answer when I began answering.