Answer: The answer is -19.
Step-by-step explanation: We are given to write the numerical coefficient of -9x.
The given expression is a monomial with unknown variable 'x'.
We know that a numerical coefficient of a monomial is a constant multiplied to the variable(s). For example, in the term -5xy, the unknown variables are 'x' and 'y' and the numerical coefficient is -5.
Therefore, in -9x, the unknown variable is 'x' and the numerical coefficient is -9.
Thus, the answer is -9.