"15 - x cubed" can be interpreted in a couple of ways. Here's how to solve each:

1. 15 minus x cubed

This means 15 minus the cube of x. The expression is:

* 15 - x³

There's no way to simplify this further unless you have a specific value for x.

2. (15 - x) cubed

This means the cube of the quantity (15 - x). The expression is:

* (15 - x)³

To expand this, you can use the binomial theorem or simply multiply it out:

* (15 - x)³ = (15 - x) * (15 - x) * (15 - x)

* = (225 - 30x + x²) * (15 - x)

* = 3375 - 525x + 15x² - 225x + 30x² - x³

* = -x³ + 45x² - 750x + 3375

In either case, you need a specific value for x to get a numerical answer.