M(ax+by+c)=0 make y subject

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

Need solving this second acceleration problem.

What are the solutions of the system? solve by graphing. y = -x^2 - 3x + 2 y = -2x + 2