They are the same length, 6 would be your answer.
Do tell if I am mistaken.
We know, Circumference of a Circle = 2πr
In mathematical form, You can write it as:
2πr₁ = 3(2πr₂)
2π(12) = 6πr₂
24π = 6πr₂
r₂ = 4 m
In short, Your Answer would be: 4 meter
Hope this helps!
The third side is 4
So Pythagorean's theorem is the following

From here we can input the following information that we know

I think its 31 but im not 100% sure :)