First, find the missing leg of ΔBCD (line BD). Using the Pythagorean Theorem, subtract (5^2)-(3^2) [25-9], and you get 16. Find the square root of 16, and you get 4 for line BD. Now, you can find AB. Add (7^2)+(4^2) [49+16], and you get 65. Now, since 65 isn't a perfect root, just use √(65)
Answer: Line AB equals √(65).
