A13-foot ladder is placed against a vertical wall of a building, with the bottom of the ladder standing on level ground 5 feet from the base of the building. how high up the wall does the ladder reach? the ladder reaches feet up the wall.

The answer is 12 because (a^2 + b^2 = c^2) 

5^2 + b^2 = 13^2  
 25  + b^2 = 169
                                                                          
169- 25 = 144    
   Sqaure root of 144 is 12 so If we plug that in..
 25 + 144 = 169 
5^2 + 12^2 =13^2 
This all checks out 
~Hope This helps