L(x, y) = 85·x + 99·y - k·(15·x^2 + 68·x·y + 3·y^2 - 5905)
L'x(x, y) = 85 - k·(30·x + 68·y) = 0 --> k = 85/(30·x + 68·y)
L'y(x, y) = 99 - k·(68·x + 6·y) = 0 → k = 99/(68·x + 6·y)
85/(30·x + 68·y) = 99/(68·x + 6·y) --> y = 1405/3111·x
15·x^2 + 68·x·(1405/3111·x) + 3·(1405/3111·x)^2 = 5905 --> x = 11.29054793
y = 1405/3111·(11.29054793) = 5.099074201
85·(11.29054793) + 99·(5.099074201) = 1464.504919