Prime numbers are numbers that can only be multiplied by 1 and themselves. But, we can see, that 9 can be multiplied by 3 and 3, making it a not-prime number (sorry I don't remember the exact name for that, I'm blanking out).
Triangle ABD and triangle CDB are similar because: -it is given that angle A and angle C are equal, both 90 -angle ABD and angle CDB are equal because they are alternate interior angles - side DB and side DB are the same length - two triangles are equal because of AAS
