For this solid we slice perpendicular to the x-axis and get squares. The slice at position x has side-length .
The volume of this slice is
The volume of the entire solid is
We could also find the volume by calculating the volume of half the solid and then multiplying by 2.
Then we get the integral expression
For symmetric objects like the one above, it's easier to find the volume by finding the volume of half the solid and then multiplying by 2. It's pretty likely you'll be asked to finish the problem by evaluating the integral, and it's easier to evaluate an integral if one of the endpoints is 0.