https://tex.z-dn.net/?f=%5Cfrac%7B1%7D%7B3%7D+%2A+%28s-2%29%3Ds%2B4+%5C%5C+%5C%5C+%5Cfrac%7Bs-2%7D%7B3%7D%3Ds%2B4+%5C%5C+%5C%5C+s-2%3D3%28s%2B4%29+%5C%5C+%5C%5C+s-2%3D+3s%2B12+%5C%5C+%5C%5C+s-3s%3D12%2B2+%5C%5C+%5C%5C+-2s%3D+14+%5C%5C+%5C%5C+%5Cboxed%7Bs%3D+-%5Cfrac%7B14%7D%7B2%7D%3D-7%7D%7D