Question

What is the coefficient of -x

Answer 1

The coefficient of "-x" is:  "-1" {"negative one"},

For "stand alone" variable, with coefficients of "1" or "-1", we often OMIT the "1" or "-1"; and just use the "+" (positive sign); or leave as written for positive; 
             and just use the " - " (negative sign; with the "1", for negative);
since any value multiplied by "1" is that same value; so the omitted "1" (or "-1"); in implied.