\(M = - 3\left( {x - 4} \right)\left( {x - 2} \right) + x\left( {3x - 18} \right) - 25\)\( = - 3\left( {{x^2} - 2x - 4x + 8} \right) + x.3x + x.\left( { - 18} \right) - 25\)

\( = - 3{x^2} + 6x + 12x - 24 + 3{x^2} - 18x - 25\) \( = \left( { - 3{x^2} + 3{x^2}} \right) + \left( {6x + 12x - 18x} \right) - 24 - 25\)

\( = - 49\)

\(N = \left( {x - 3} \right)\left( {x + 7} \right) - \left( {2x - 1} \right)\left( {x + 2} \right) + x\left( {x - 1} \right)\) \( = x.x + x.7 - 3.x - 3.7 - \left( {2x.x + 2x.2 - x - 1.2} \right) + x.x + x.\left( { - 1} \right)\)

\( = {x^2} + 7x - 3x - 21 - 2{x^2} - 4x + x + 2 + {x^2} - x\)

\( = \left( {{x^2} - 2{x^2} + {x^2}} \right) + \left( {7x - 3x - 4x + x - x} \right) - 21 + 2\)

\( = - 19\)

Vậy \(M = - 49;N = - 19 \Rightarrow M - N = - 30\) .