`a, ( - 15 )/x = 25/45`
`⇔ x . 25 = ( - 15 ) . 45`
`⇔ x . 25 = - 675`
`⇔ x = - 675 : 25`
`⇔ x = - 27`
Vậy `, x = - 27 .`
`b, 21 : 245 = ( - 15 ) : 7x`
`⇔ 21 . 7x = 245 . ( - 15 )`
`⇔ 147x = - 3675`
`⇔ x = - 3675 : 147`
`⇔ x = - 25`
Vậy `, x = - 25 .`