What is the slope intercept form of 5x-y=9

The   " y = 5x − 9 " .

Explanation: 

Given:  " 5x − y = 9 " ; 

Write  in slope intercept form:

  " y = mx + b " ;

         in which "y" is on the left-hand side of the equation, "on its own" (that is, with no "written coefficient")—other than the "implied coefficient of 1 " ;
  "m" is the number, or fraction (positive or negative) representing the slope, if there is one—and "m" is the coefficient of "x"  and "b" is the "y-intercept", if there is one—and may be positive or negative.

Given:

  5x − y = 9 ;
 
Multiply the entire equation by "-1" ; to get rid of the "− y " ;
 
   -1 * {5x − y = 9 } ;

   -5x + y = -9 ;

↔  Rewrite as:  y + (-5x) = -9 ;

↔  Rewrite as:  y − 5x  =  -9 ;

           Add  "5x" to both sides of the equation; to isolation "y" on the left-hand side of the equation:

             y − 5x + 5x  =  -9 + 5x ; 

to get:    y = -9 + 5x  ;

↔ Rewrite as:  y = 5x + (-9) ;

↔ Rewrite as:  y = 5x − 9 ;

This equation is written is "slope-intercept form"; that is:

       " y = mx + b "  ; in which;
                                      m = the slope = 5 ; 
                                      b = the y-intercept =  -9 ;