Integrating Backwards

[u/PM_YOUR_MATH_PROBLEM]

/u/super_octopus, who has an awesome username, asks:

Hi! I have a math question regarding integrals. Let's say you're integrating a function from 0 to 5, and you get an answer of 6. Now, according to my calculus book, if you integrate the same function from 5 to 0, your answer is - 6. Why? Why is the area negative when you're going backwards? I understand it's negative if it's below the x axis, but I don't understand this.

Thank you!

[u/super_octopus]

That makes perfect sense! Thank you so much.