Answer: 20 ;)
Step-by-step explanation:
Y = a(b^x)
y=mx + b is the equation of a line. It is a linear function because it has a constant rate of change.
y = a(b^x) is an exponential function because it has has an exponent and its rate of change is not constant
The answer to your question is 81,300.
I'm sorry but the solution you proposed was a bit hard to follow, so I'll just post my solution: we convert "2 and 1/2" to a single fraction:

So, we know that 5/2 gallons cost 60 dollars, which means

To find the cost of a single gallon, we have to solve the equation for g:

The goal of all algebra equations is to isolate the variable. In 3 step you should first make sure both variables are on the same side. To do that make sure you use the inverse.
For example:
6a + 9 = 3a + 15
because 3a is positive you're going to want to subtract it.
6a + 6 = 3a + 15
-3a -3a
_______________
3a + 6 = 15
Now it's a 2 step equation and you solve it on from there.