Answer:
<u>Yes. I agree with his claim because the equation relating the miles left to walk and the number of miles left to walk is :</u> 
Step-by-step explanation:
Given :
- <em>It's a 21 - mile walkathon</em>
- <em>The rate at which he is moving is 3 miles per hour.</em>
Let the number of miles walked be
;
∴,
The number of miles '
' left to walk will be :
<em>Thus the time '
' for which he walked :</em>
:
∴
From these equations , we get :

Thus , when we compare this with
,

<u>Thus this is an equation of a Line with slope -3.</u>