Answer:
it's answer is 3x - 3
y + x - 8 + 8x + 4 = 12x - 7
y + 9x - 4 = 12 x - 7
y = 12x -9x - 7 +4
y = 3x - 3
hope it helps you
[tex]\Large \boxed{\sf 3x-3}[/tex]
Step-by-step explanation:
Perimeter is sum of all sides
Subtract both given sides from perimeter to get the missing side
[tex](12x-7)-( \ (x-8)+(8x+4) \ )=3x-3[/tex]
