In math, what does justify your answer using complete sentences mean?

For the rule of 78, for a 12-month period, the last term in the sequence is 12 and the series sums to 78. for an 10 month period, the last term is and the series sum is . for a 15 month period, the last term is and the series sum is . for a 20 month period, the last term is and the series sum is

Ineed i been stuck on this question since yesterday

Graph the equationy=x^2 -[tex]y = x^{2} - 2[/tex]