Salut !
[tex][12 + 3 \times (2x + 1)] = 24[/tex]
[tex]3 \times (2x + 1) = 24 - 12[/tex]
[tex]3 \times (2x + 1) = 12[/tex]
[tex]2x + 1 = 12:3[/tex]
[tex]2x + 1 = 4[/tex]
[tex]2x = 4 - 1[/tex]
[tex]2x = 3[/tex]
[tex] = > x = \frac{3}{2} [/tex]
..................