Answer: AB = 15 units long.
Explanation:
The triangles are identical.
You can infer this from both triangles sharing the side of BD and AD and CD marked as equal in length.
When triangles are identical, all three sides and angles are identical in length and measure, respectively.
Knowing this, we can set the sides of "5x" and "4x + 3" equal to each other in an equation to solve for x:
5x = 4x + 3
x = 3
When x equals 3, multiply x by 5 to find the side length of AB.
AB = 15 units long.