Because the 2 triangles are similar, find the ratio of 2 known sides.
Side XY = 10
Side AB = 25
25 / 10 = 2.5
This means the larger triangle is 2.5 times larger than the smaller one.
Now find the side of the smaller triangle, that would be the same side as X on the larger triangle and multiply that by 2.5.
XZ is the same side as AC
So X = 6 x 2.5
X = 15