a)
`2x+5=17`
`⇔2x=12`
`⇔x=6`
b)
ĐKXĐ `x\ne±5`
`(x+5)/(x-5)-(x-5)/(x+5)=20/(x^2-25)`
`⇒(x+5)^2-(x-5)^2=20`
`⇔x^2+10x+25-x^2+10x-25-20=0`
`⇔20x-20=0`
`⇔x-1=0`
`⇔x=1`
c)
`(x+1)(x+2)=(3-x)(x+2)`
`⇔(x+1)(x+2)-(3-x)(x+2)=0`
`⇔(x+2)(x+1-3+x)=0`
`⇔(x+2)(2x-2)=0`
`⇔`\(\left[ \begin{array}{l}x+2=0\\2x-2=0\end{array} \right.\)
`⇔`\(\left[ \begin{array}{l}x=-2\\x=1\end{array} \right.\)