Question 404138
Remember, the Pythagorean theorem states that a²+b²=c².  
a and b are the legs.  The order doesn't matter.  c is the hypotenuse, or the side opposite the right angle.  It is also the longest side in a triangle. 
So let's substitute a=5 and b=12 into the Pythagorean theorem and solve for c, the hypotenuse.
5²+12²=c²
25+144=c²  Simplify squares.
169=c²     Add like terms.
√169=√c²   Take the square root of both sides to isolate the c.
13=c       Simplify
So c, or the hypotenuse, is 13 :)