In this case, we'll have to carry out several steps to find the solution.
Step 01:
Data
Table
Step 02:
We must analyze the data table to find the solution.
constant rate:

y = mx + b
(y - y1) = m (x - x1)
(y - 450) = -6 (x - 10)
y = -6x + 60 + 450
y = -6x + 510
if x = 0 (initial time)
y = -6*(0) + 510
y = 0 + 510
y = 510
The answer is:
510 gallons