the sum of the cube of a number and 12

the sum of the cube of a number and 12

The phrase a number means an arbitrary variable, let's call it x.

The cube of a number means we raise x to the power of 3:
x^3

Finally, we take the sum of x^3 and 12. Meaning, we add 12 to x^3. This is our final algebraic expression.
x^3 + 12