Can someone help me? I'm terrible at math lol

Answer:
D
Step-by-step explanation:
Following Pythagorean Theorom, we know a^2 + b^2 = c^2. We know b and c. a^2 + 5^2 = 13^2.
a^2 + 25 = 169
a^2 = 144
a = √144
a = 12
So, the answer is 12, which means we pick D