a: \(B=\left(\dfrac{21}{\left(x-3\right)\left(x+3\right)}+\dfrac{x-4}{x-3}-\dfrac{x-1}{x+3}\right):\left(1-\dfrac{1}{x+3}\right)\)
\(=\dfrac{21+x^2-x-12-x^2+4x-3}{\left(x-3\right)\left(x+3\right)}:\dfrac{x+3-1}{x+3}\)
\(=\dfrac{3x-9}{\left(x-3\right)\left(x+3\right)}\cdot\dfrac{\left(x+3\right)}{x+2}=\dfrac{3}{x+2}\)
b: |2x+1|=5
=>2x+1=5 hoặc 2x+1=-5
=>x=2(nhận) hoặc x=-3(loại)
Khi x=2 thì B=3/4
c: để B=-3/5 thì x+2=-5
=>x=-7