# How do you write this as a variable expression "the sum of -3 and a number?

$x - 3$
In a variable expression, "a number" is represented by a variable, (letter). I this case I chose to use $x$ as "a number". So the sum of a number $\left(x\right)$ and $\left(- 3\right)$ can be written as:
$x + \left(- 3\right)$ which simplifies to:
$x - 3$