Idon't understand pythagorean theorem

So the Pythagorean Theorem is a²+b²=c² pretty much all you do is plug in what you know. So for number 3 you would plug in the numbers that you know into the formula. 
20²+b²=25²
BD isn't needed in the formula because it's not apart of the legnth we are looking at so you dont need to worry about that. 
Now do what the formula says.. 
400+b²=625
then you would subtract
625-400= 225
and "un-square" the answer by finding the square root 
√225 = 15 
AB= 15
you can always check your answer 
20²+15²=25²
400+225=625