Answer:
5
Step-by-step explanation:
Use the Pythagorean Theorem
a²+b²=c² plug in the info you have
a²+12²=13² (c is always the side opposite from the right angle)
Solve for a
a²+144=169
-144 -144
a²=25 take the square root of both sides
√a²=√25
a=5
