Hi!
Perimeter is all of the side lengths added up, therefore:
3x + 12 + 3x + 12 + 3x + 12 = 12x - 6
Add like terms:
9x + 36 = 12x - 6
Add 6 to both sides:
9x + 42 = 12x
Subtract both sides by 9x:
42 = 3x
Divide both sides by 3:
14 = x
Plug this into the equation 3x + 12:
3(14) + 12
Multiply 3 and 14:
42 + 12
Add 42 and 12:
54
The side length is 54 cm.