u/Legitimate-Copy3076

When using the formula for the washer method, I assumed that, like with the previous formulas I have learned in the unit, it was the upper function minus the lower function. However, the solution for this problem has the lower function minus the lower function.

Is there an explanation for this? I am confused, is the correct order for the formula actually the lower function minus the upper function?

The second photo is my work, I don't really think it's necessary for my specific question but it might provide some additional context. Sorry if it's messy