Since you know that BD=24, you also know that both segments BC and CD have the same value since they're congruent: 12. You also were given the value of PC. Using Pythagorean Theorm, Substitute 12 for a and 13 for c, so the equation would say
12^2+b^2=13^2
Simplify to get
144+b^2=169
Now subtract 144 from 169 and you get 25. That is b^2. All you have to do now is square root 25 to isolate b.
