実数って1行目に断ってるぞ…
あと、一応Wolfram alphaのURL乗っけとくわ
https://www.wolframalpha.com/input/?i=solve+a%2Bc%3D-4%2F3%2C+b%2B4ac%2Bd%3D-2%2C+ad%2Bbc%3D4%2C+bd%3D1