The difference between the product of 4 and a number and the square of a number

The difference between the product of 4 and a number and the square of a number

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

The product of 4 and a number:
4x

The square of a number means we raise x to the power of 2:
x^2

The difference between the product of 4 and a number and the square of a number:
4x - x^2