Trả lời :
B2 :
$a.\\\begin{array}{I}x-\frac{3}{7}=\frac{1}{4}\\\Rightarrow x=\frac{1}{4}+\frac{3}{7}=\frac{19}{28}\end{array}$
Vậy $x=\frac{19}{28}$
$b.\\\begin{array}{I}3x-\frac{1}{2}=0\\\Rightarrow 3x=0+\frac{1}{2}=\frac{1}{2}\\\Rightarrow x=\frac{1}{2}:3=\frac{1}{6}\end{array}$
Vậy $x=\frac{1}{6}$