In the diagram below, bc is an altitude of abd. to the nearest whole unit, what is the length of cd?

For the answer to the question above, I'll show my solution on how you can solve your problem.
AC² + BC² = AB² 
5² + 12² = AB² 
25 + 144 = AB² 
169 = AB² 
13 = AB 

Triangle ABC is proportional to triangle ADB. 

AC = AB 
AB    AD 

5 = 13 
13  AD 

5AD = 169 

AD = 169/5 

AD = 33.8 

AD = AC + CD 

33.8 = 5 + CD 
So the answer is

CD = 28.8 

X= 49; x = 4step 1: foilstep 2: equal both to 0step 3: find x individually