Tìm x , biết: (a) 3 x − 7/8 = 5/2 ; (b) 2 x ( 3 x − 1 ) − 6 x ( x + 2 ) = 42 .

a) \(\frac{{3x - 7}}{8} = \frac{5}{2}\)

\(2.\left( {3x - 7} \right) = 8.5\)

\(6x - 14 = 40\)

\(6x = 54\)

\(x = 9\)

Vậy \(x = 9\).

b) \[2x\left( {3x--1} \right)--6x\left( {x + 2} \right) = 42\]

\(6{x^2} - 2x - 6{x^2} - 12x = 42\)

\( - 14x = 42\)

\(x = - 3\)

Vậy \(x = - 3\).