`a) 43 + (9 – 21) = 317 – (x + 317)`
`43 + -12 = 317 - (x + 317)`
` 31 = 317 - x + 317 `
` 31 = ( 317 - 317) + -x`
` 31 = -x`
⇒`-x = -31`
b) `6x + 4x = 2010`
`6.x + 4.x = 2010`
`( 6+4) . x = 2010`
` 10 . x = 2010`
` x = 2010 : 10`
` x = 201`
d)`5x - 3x - x = 20`
` 2x- x = 20`
`=> x=20`