2(cost of gallon of diesel) - $3.25 = (cost of gallon of gas)
This can be written down as 2d - 3.25 = 3.95, which is provided in the text.
If you switch sides then you get 3.95 + 3.25 = 2d, which is then 7.20 = 2d, or 3.60 = d.
Now that we have the price of a gallon of Diesel and you can check it buy just putting all the numbers in the equation.