the perimeter, as you already know, is the sum of all the sides.
14)
[tex]\bf (x+3)+(2x+4)+(y)=\stackrel{perimeter}{6x}\implies 3x+7+y=6x
\\\\\\
7+y=3x\implies y=3x-1[/tex]
15)
a rectangle has twin widths and twin lengths, therefore
[tex]\bf \stackrel{width}{(3x-3)}+\stackrel{width}{(3x-3)}+\stackrel{length}{(y)}+\stackrel{length}{(y)}=\stackrel{perimeter}{10x+6}
\\\\\\
6x-6+2y=10x+6\implies -6+2y=4x+6\implies 2y=4x+12
\\\\\\
y=\cfrac{4x+12}{2}\implies y=\cfrac{4x}{2}+\cfrac{12}{2}\implies y=2x+6[/tex]
