The sum of 3 times the square of a number and negative 7

The sum of 3 times the square of a number and negative 7

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

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

3 times the square of a number:
3x^2

The sum of 3 times the square of a number and negative 7
3x^2 - 7