Explicação passo-a-passo:

PA => ( a ; b ; c )

[tex]b = \frac{a \: + \: c}{2} [/tex]

[tex]2x - 1 = \frac{x \: + \: (5x + 7)}{2} [/tex]

[tex]2x - 1 = \frac{6x \: + \: 7}{2} [/tex]

[tex]6x + 7 = 2.(2x - 1)[/tex]

[tex]6x + 7 = 4x - 2[/tex]

[tex]6x - 4x = - 2 - 7[/tex]

[tex]2x = - 9[/tex]

[tex]x = - \frac{9}{2} [/tex]