a) Ta có: $\left( a+b \right)-\left( b+c \right)+\left( c+a \right)=5-16+\left( -19 \right)$
$a+b-b-c+c+a=-30$
$2a=-30$
$a=-30:2$
$a=-15$
Khi đó: $-15+b=5$$\Rightarrow b=5+15$$\Rightarrow b=20$
$c+\left( -15 \right)=-19$$\Rightarrow c=-19-\left( -15 \right)$$\Rightarrow c=-19+15$$\Rightarrow c=-4$
Vậy a = -15; b = 20; c = -4
b) Ta có: $\left( a-b \right)+\left( b-c \right)+\left( c+a \right)=9+\left( -10 \right)+11$
$a-b+b-c+c+a=10$
$2a=10$
$a=10:2$
$a=5$
Khi đó: $5-b=9\Rightarrow b=5-9\Rightarrow b=-4$
$c+5=11\Rightarrow c=11-5\Rightarrow c=6$
Vậy a = 5; b = -4; c = 6