`a)`
` 2/3 * x - 1/2 * x = 5/12`
`\to x ( 2/3 -1/2 ) = 5/12`
`\to x * 1/6 = 5/12`
`\to x = 5/12 : 1/6`
`\to x =5/2`
Vậy `x =5/2`
`b)`
` ( 2 4/5 * x -50) : 2/3 =51`
`\to ( 2 4/5 * x - 50) = 51 * 2/3`
`\to 14/5 * x - 50 = 34`
`\to 14/5 *x = 84`
`\to x = 30`
Vậy ` x= 30`