a) $N = -\left \{ -\left ( a + b \right ) - \left [ \left ( a - b \right ) - \left ( a + b \right ) \right ] \right \}$
$N = -\left [ -\left ( a + b \right ) - \left ( a - b - a - b \right ) \right ]$
$N = -\left ( -a - b + 2b \right )$
$N = a - b$
b) Thay $a = -5; b = -3$ ta có:
$N = a - b = -5 - \left ( -3 \right ) = -5 + 3 = -2$