The answer is the first one
8.25g+65=683.75
As you can see in the question it says that each gallon is $8.25. Since each gallon is $8.25 then the g for gallons would go after 8.25 in the equation.
Since he but one irrigation unit for $65, then we only add 65. Lastly, the total cost is $683.75. This means that this number goes behind the equal sign.
The equation is 8.25g + 65 = 683.75