We can work through this problem algebraically, with steps. 

So, first you can identify the fact that the volume of water that still needs to be added to the pool is dependent on the time elapsed since Isabella started filling the pool. She fills her pool at a constant rate, and we need to make an equation to model this situation. The rate at which Isabella fills the pool is the same thing as the slope of the equation we make, in litres per minute. 

The goal of this question, then, is to find the slope of the equation modelling this situation. We can use the following information:

m = Δy / Δx

m = (94 - 184) / (7 - 2)

m = -90 / 5

m = -18 litres per minute

However, this is the water left in the pool with respect to time. It's negative because the amount of water left in the pool is going down. The rate at which Isabella fills the pool is the same as this number, but not negative, because the amount of water in the pool increases. 

So, Isabella fills her pool at 18 litres/minute. 

Thanks. Your answer is very clear, even to this math-challenged person.

But how does one set up the equation to find something other than the rate? For example, the  volume of the pool is or how many liters she started with in the pool (assuming it's a value greater than zero)?

The total capacity of the pool (it's volume with or without water) would be constant.  The amount she started with, there's not enough information supplied for that.  Would require for example knowing the total water volume to be present.