To answer this question you must use Pythagorean theorem
a and b are the legs (the sides that form a perpendicular/right angle)
c is the hypotenuse (the side opposite the right angle)
In this case...
a = 12
b = unknown
c = 13
^^^Plug these numbers into the theorem
solve for b
+ 144 = 169
= 25
To remove the square from b take the square root of both sides to get you...
b = 5
Hope this helped!
~Just a girl in love with Shawn Mendes