`a) x × 1/2 + x × 3/4 = 6/7`
`=> x × (1/2 + 3/4) = 6/7`
`=> x × 5/4 = 6/7`
`=> x = 6/7 : 5/4`
`=> x = 24/35.`
`b) 2/5 × x - 1/4 × x = 1/3`
`=> x × (2/5 - 1/4) = 1/3`
`=> x × 3/20 = 1/3`
`=> x = 1/3 : 3/20`
`=> x = 20/9.`
`c) 5/6 × x = 4/5`
`x = 5/6 : 4/5`
`x = 25/24.`
`d) 12/17 : x = 1/2`
`=> x = 12/17 : 1/2`
`=> x = 24/17.`