We just need to know the method that is used to relate one value to another. Calculus is a branch of mathematics that we use to describe the behavior of functions. We make that determination by differentiating the function. Derivatives represent a rate of change. This information allows us to draw a graph of the function and to describe its behavior.
Integral calculus is used to determine which function, if any, changes in a specified manner.
We make this determination by trying to find a function that, when differentiated, yields the specified function. This is called integration. This is not a single function but rather an entire family of functions with the desired property. The problem above is a simple one. You have used other techniques to determine answers to more difficult questions.