Can someone explain this? i don't understand a thing. chapter on functions.

This passage is about the domain of a function. The domain of a function is the set of all points that you can give to the function as an input.

Points not belonging to the domain are not good inputs for the function. In this example, 3 is not a good input, because if you try to evaluate the function at , i.e. if you plug 3 in place of x, you get

which is undefined, because you can't divide by 0.

So, when defining this function, you have to specify , so that the function will be defined for every other input.

the answer is 62.8 cubic units