Ариф.прорессия S4=-28 S6=58 Sn-?

Решите задачу:

$S_4=-28\; ;\; \; S_6=58\; ;\; \; S_{n}=?\\\\S_{n}= \frac{2a_1-(n-1)d}{2}\cdot n \; ;\\\\S_4=\frac{2a_1-3d}{2}\cdot 4=4a_1-6d=-28\\\\S_6=\frac{2a_1-5d}{2}\cdot 6=6a_1-15d=58\\\\ \left \{ {{2a_1-3d=-14} \atop {6a_1-15d=58}} \right. \; \left \{ {{6a_1-9d=-42} \atop {6a_1-15d=58}} \right. \; \left \{ {{6d=-100} \atop {6a_1-15d=58}} \right. \\\\ \left \{ {{d=-\frac{50}{3}} \atop {6a_1+15\cdot \frac{50}{3}=58}} \right. \; \left \{ {{d=-\frac{50}{3}} \atop {a_1=-32}} \right.$

$S_{n}= \frac{2(-32)-(n-1)(-\frac{50}{3})}{2} \cdot n= \frac{-64+(n-1)\frac{50}{3}}{2} \cdot n= \frac{-192+50(n-1)}{6}\cdot n$