a) I 9 + b I = 21 b) I b - 7 I = (-6) + 15

`a)`

`|9 + b| = 21`

`=>` $\left[ \begin{array}{l}9 + b = 21\\9 + b = -21\end{array} \right.$

`=>` $\left[ \begin{array}{l}b = 21 - 9\\b = -21 - 9\end{array} \right.$

`=>` $\left[ \begin{array}{l}b = 12\\b = -30\end{array} \right.$ 

`Vậy: b ∈ {12; -30}`

`b)`

`|b - 7| = (-6) + 15`

`|b - 7| = 9`

`=>` $\left[ \begin{array}{l}b - 7 = 9\\b - 7 = -9\end{array} \right.$

`=>` $\left[ \begin{array}{l}b = 9 + 7\\b = -9 + 7\end{array} \right.$

`=>` $\left[ \begin{array}{l}x = 16\\b = -2\end{array} \right.$ 

`Vậy: b ∈ {16; -2}`