You just need to plug those expressions inside the formula: it doesn't matter if they're expressions involving a variable instead of plain numbers: the formula becomes
![V = \pi (x+8)^2(2x+3)](https://img.qammunity.org/2019/formulas/mathematics/middle-school/jf7lqpj3lepseb5hxacglth50aud0r4exa.png)
If you want, you can simplify it by expanding the square and then multiply the two parenthesis:
![V = \pi (x^2+16x+64)(2x+3) = \pi (2 x^3 + 35 x^2 + 176 x + 192)](https://img.qammunity.org/2019/formulas/mathematics/middle-school/zdfio0jhgwtciqj0x4yi1yq4pc9kqwblig.png)