Uniformly Minimal Variance - Estimation

#1
Hi there, I am a statistics student and the university taught us Uniformly Minimal Variance, The question is very basic but still I can't see the connection between what is taught to the solution of this problem.

Its given that X and Y are estimations with no variance to the parameter 0

The questions is what is the condition that has to be met so the estimation (aX+bY) would have no variance to 0

I have no idea how to approach this subject at all. I would be happy if on top of the answer you can direct me to a good studying place or a good article on the subject.

Thank you in advance.