`a,`
`8x + 15 - 3x = -400`
`=> (8x - 3x) = -400 - 15`
`=> 5x = -415`
`=> x = -415 : 5`
`=> x = -83`
`b,`
`-32x + 12x - 5x = 900`
`=> (-32 + 12 - 5) . x = 900`
`=> -25 x = 900`
`=> x = 900 : -25`
`=> x = -36`
`c`
`(x + 2) . (x - 5) = 0`
`=>` \(\left[ \begin{array}{l}x + 2 = 0\\x - 5 = 0\end{array} \right.\)
`=>` \(\left[ \begin{array}{l}x=-2\\x=5\end{array} \right.\)
`d,`
`(x^2 - 25) . (x^2 + 25) = 0`
`=>` \(\left[ \begin{array}{l}x^2 - 25 = 0\\x^2 + 25 = 0\end{array} \right.\)
`=>` \(\left[ \begin{array}{l}x^2 = 25\\x^2 = -25 (KTM)\end{array} \right.\)
`=> x^2 = 25`
`=>` \(\left[ \begin{array}{l}x = 5\\x=-5\end{array} \right.\)
