Answer: I think its $176.40
Step-by-step explanation:
The area would be 21
Feet would be = 50.4 (21 x 2.4)
Then it would cost $176.40 (50.4 x $3.50)
I might be wrong, I'm not entirely sure.
 
        
                    
             
        
        
        
Answer is choice DThis is the only answer choice with a negative slope, so we don't even need to look at the point really. Though if you wanted, you can confirm that the point (1,-3) is on the diagonal line. This point is shown in red in the attached image.
To find that the slope is -6/5, you pick two points on the diagonal line and use the slope formula. Two points you can use are (1,-3) and (-4,3). 
The slope formula is m = (y2-y1)/(x2-x1)
Note: The negative slope is because we move downhill as we move from left to right along the diagonal line.