Question 1135164
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The "solution" by @mathsolverplus is not only incomplete - it is incorrect.


The problem formulation is incorrect,  too.


In order for the polynomial of the degree 3 could be identified / (found) by an unique way,  its values should be given at  4  points,


while the condition provides its values only at  3  points.



So, &nbsp;both the condition and the &nbsp;"solution" &nbsp;by &nbsp;@mathsolverplus &nbsp;&nbsp;<U>ARE &nbsp;DEFECTIVE</U>.


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It is just, probably, couple of weeks I observe an activity of @mathsolverplus at this forum.


Regarding this activity, I can resume that for routine problems @mathsolverplus, probably, can provide valid solutions 
(although not always).


But for the problems "one step away from routine" the probability he provides incorrect answers becomes dangerously high.


Under supervision and correction from other tutors in this forum, it still can be kept in some reasonable frames 
(although, I personally am not very happy to clean after him . . . ).


But as a self-standing tutor at his own web-site, without supervising and corrections from other tutors he is not able 
to provide error-proven (error-free) solutions on stable basis.


Keep it in your mind if/when you will consider to follow him at his web-sites.


You will be at constant risk of getting wrong solutions.