If the perimeters are equal, you can form an equation to find x:
x + x + 12 + x + x + 12 = x + 11 + x + 11 + x + 16
4x + 24 = 3x + 38
- 3x
x + 24 = 38
- 24
x = 14
Now you can substitute the x value into the perimeter:
(4 × 14) + 24
56 + 24 = 80 cm
(3 × 14) + 38
42 + 38 = 80 cm
So the perimeter would be 80 cm.
I hope this helps!