Подстановка условия
[src](3*m - 4*n)*k - m*(6*n + 5*k) - n*k - (-1)*(6*m)*n при k = 3
(3*m - 4*n)*k - m*(6*n + 5*k) - n*k - (-1)*(6*m)*n
$$- -1 \cdot 6 m n + - k n + k \left(3 m - 4 n\right) - m \left(5 k + 6 n\right)$$
(3*m - 4*n)*(3) - m*(6*n + 5*(3)) - n*(3) - (-1)*(6*m)*n
$$- -1 \cdot 6 m n + - (3) n + (3) \left(3 m - 4 n\right) - m \left(5 (3) + 6 n\right)$$
(3*m - 4*n)*3 - m*(6*n + 5*3) - n*3 - (-1)*(6*m)*n
$$- -1 \cdot 6 m n + - 3 n + - m \left(6 n + 3 \cdot 5\right) + 3 \left(3 m - 4 n\right)$$
-15*n + 9*m - m*(15 + 6*n) + 6*m*n
$$6 m n - m \left(6 n + 15\right) + 9 m - 15 n$$