Answer:
A) 2a + 6
Step-by-step explanation:
(5a + 3) + (2a + 3) + x = 9a + 12
Add both side lengths plus the unknown side to equal the perimeter.
7a + 6 + x = 9a + 12
-7a -6 -7a -6 Subtract 7a + 6 on both sides
x = 2a + 6 This is the missing side length.
