X will be red pens because they are mentioned in both cases.
84 = red pens (x) + blue pens (6 times red pens(x)) + black pens (red pens(x) + 4)
[tex]\begin{gathered} 84=x\text{ + 6x + \lparen x+4\rparen} \\ 84=\text{ x + 6x + x+ 4} \\ 84=\text{ 8x + 4} \\ 84\text{ - 4= 8x} \\ 80=\text{ 8x} \\ \frac{80}{8}=\text{ x} \\ \\ 10=x \\ \\ Then,\text{ for black pens the equation will be: } \\ x\text{ + 4=} \\ 10\text{ + 4=} \\ 14\text{ black pens. } \end{gathered}[/tex]
