Create an expression to find the time for the given distance and rate, and find the value of the expression.
d = 84 feet; r =12 feet/second
the time is seconds?

Note the equation/formula:

      
         d * r = t  ;

  in which:  

        d = distance = 84 ft.
        r  = rate = 12 ft / sec
        t  = time = ?

To find the "time" we can rearrange the equation:

       d = r * t  ;   

   Divide EACH SIDE of the equation by "r" ;  to isolation "t" on one side of the equation; to create an expression to find the time for the given distance and rate; which we will then do by plugging in our given values for "d" (distance) and "r" (rate) ;

     d / r = (r * t) / r ;  

 to get :   d / r = t ; 

         ↔   t = d / r  ;

Now, plug in our given values for "d" and "r" ; to solve for "t" ;

        →    t = 84 ft / (12 ft / sec) ; 
   
                  =  84 ft   ÷    ;
    
                  =  84 ft  *   ;
  
                  =  {   }  sec. ; 
  
                    →   t    =   7 sec.   {that is;  "7 seconds" }