where Q is the quaternion group. Thanks !
Here are my thoughts : With the Lagrange's theorem, we have that the order of the subgroup divides the order of the group. But there is a single element that has order 2 (b for example), thus you have that there exists a unique subgroup of order 4 generated by a (but is it stated without proof because it's straightforward?) and thus you have that b^2 = a^2 because b is the only element that has order 2. And i don't know how to proof that aba = b
