a)
M(x) = $3x^{3}$ - 2x + $4x^{2}$ - x + 5
M(x) = $3x^{3}$ + $4x^{2}$ - 2x - x + 5
M(x) = $3x^{3}$ + $4x^{2}$ - 3x + 5
N(x) = $2x^{2}$ - x + $3x^{3}$ - $3x^{2}$ + 9
N(x) = $3x^{3}$ + $2x^{2}$ - $3x^{2}$ - x + 9
N(x) = $3x^{3}$ - $x^{2}$ - x + 9
b)
M(x) + N(x) = $3x^{3}$ + $4x^{2}$ - 3x + 5 + $3x^{3}$ - $x^{2}$ - x + 9
M(x) + N(x) = $3x^{3}$ + $3x^{3}$ + $4x^{2}$ - $x^{2}$ - 3x - x + 5 + 9
M(x) + N(x) = $6x^{3}$ + $3x^{2}$ - 4x + 14
Học tốt.