a: ĐKXĐ: x<>3; x<>-3
b: \(B=\dfrac{5x-15+3x+9-5x-3}{\left(x-3\right)\left(x+3\right)}=\dfrac{3x-9}{\left(x-3\right)\left(x+3\right)}=\dfrac{3}{x+3}\)
c: Khi x=6 thì \(B=\dfrac{3}{6+3}=\dfrac{3}{9}=\dfrac{1}{3}\)
d: Để B nguyên thì \(x+3\in\left\{1;-1;3;-3\right\}\)
hay \(x\in\left\{-2;-4;0;-6\right\}\)