well, i'll tell u the procedure.

 

firstly distribute the sigma sign over all the terms,

and then apply formulae for the sum of first n terms and sum of squares of the first n terms.

 

 

 k² +3   k +  3 =LHS

 

n(n+1)(2n+1)/6 + 3. n(n+1)/2 +3n =LHS

 

and on further simplifying u might get the answer