\(a,\dfrac{x+1}{99}+\dfrac{x+2}{98}+\dfrac{x+3}{97}+\dfrac{x+4}{96}=-4\\ \left(\dfrac{x+1}{99}+1\right)+\left(\dfrac{x+2}{98}+1\right)+\left(\dfrac{x+3}{97}+1\right)+\left(\dfrac{x+4}{96}+1\right)=-4+4\\ \dfrac{x+100}{99}+\dfrac{x+100}{98}+\dfrac{x+100}{97}+\dfrac{x+100}{96}=0\\ \left(x+100\right)\left(\dfrac{1}{99}+\dfrac{1}{98}+\dfrac{1}{97}+\dfrac{1}{96}\right)=0\\ x+100=0\\ x=-100\)
\(b,\dfrac{3}{1.4}x+\dfrac{3}{4.7}x+...+\dfrac{3}{31.34}x=33\\ x\left(\dfrac{3}{1.4}+\dfrac{3}{4.7}+...+\dfrac{3}{31.34}\right)=33\\ x\left(1-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+...-\dfrac{1}{31}+\dfrac{1}{34}\right)=33\\ x\left(1-\dfrac{1}{34}\right)=33\\ x.\dfrac{33}{34}=33\\ x=33:\dfrac{33}{34}\\ x=34\)