Answer:
20 units (I used units because that's what i was taught when there isn't a specific unit)
Explanation:
If the triangles are congruent, that means that everything is congruent/the same. So what we do is, we set both equations equal to each other.
6x - 3 = 4x + 9
And now we simplify.
6x - 3 = 4x + 9
-4x -4x
--------------------
2x - 3 = 9
+3 +3
-------------------
2x = 12
÷2 ÷2
---------------
x = 6
To find the perimeter, just multiply 6 by 4 since there's four sides. 6 x 4 = 20