Answer:\frac{s-a}{n-1} 
Step-by-step explanation: 
formula, s=a+(n-1)d
1. get rid of the a on d's side by subtracting it from both sides.  
s=a+(n-1)d   becomes s-a=(n-1)d
2. get rid of (n-1) by dividing it from both sides. s-a=(n-1)d   becomes   
 \frac{s-a}{n-1} =d  
 
        
             
        
        
        
The equation that you are looking for is y = -2x + 3
        
             
        
        
        
Y = -2m -1 is your equation