The perimeter of a triangle is the sum of all sides' lengths.
Thus,
[tex]P=6+4x+(3x+2)=7x+8.[/tex]
Given [tex]P=57\ cm,[/tex] you get
[tex]7x+8=57,\\ \\7x=57-8,\\ \\7x=49,\\ \\x=\dfrac{49}{7},\\ \\x=7\ cm.[/tex]
Answer: x=7 cm.
