your equation is:
a² + b² = c²
So, plug in what you know.
5² + 12² = c²
5² means 5*5, so your "a" is equal to 25
12² means 12*12, so your "b" is equal to 144
25 + 144 = c²
to this part, add a and b. you'll get 169.
169 = c²
next, use your calculator and square both sides.
√169 = √c²(you don't really have to do anything with the c, just get rid of the ²)
squaring both sides will get rid of the ² in c, and will give you your new number:
c = 13