Tìm x, biết : (a) 3(x−7)+5(x+2)=13.

a) \(3\left( {x - 7} \right) + 5\left( {x + 2} \right) = 13\)

\(3x - 21 + 5x + 10 = 13\)

\(8x - 11 = 13\)

8x = 24

x = 3.

Vậy x = 3.

b) \(\left( {x - 4} \right)\left( {x + 1} \right) - \left( {x + 2} \right)\left( {x - 3} \right) = 10\)

\(\left( {{x^2} - 3x - 4} \right) - \left( {{x^2} - x - 6} \right) = 10\)

\({x^2} - 3x - 4 - {x^2} + x + 6 = 10\)

−2x + 2 = 10

x = − 4

Vậy x = − 4.