Answer:
50
Explanation:
Since M is the midpoint of XY, this means that XM and MY must be equivalent from the definition of midpoint.
So:

The entire length of the segment (XY) is:

We know that XM is equivalent to MY. Substitute MY for XM:

Combine like terms;

We are told that YM (or MY) is 25. So:

Multiply:

So, the length of XY is 50.
And we're done!