In the diagram below, AX is parallel to BY, AB is perpendicular to BY, and AZB is an isosceles right triangle. If AB=7 and XY=25, what is the length of AX?
[asy]
size(7cm);
draw((0,0)--(0,7)--(17/2,7)--(-31/2,0)--(0,0));
draw((0,0)--(-7/2,7/2)--(0,7));
label("B",(0,0),S);
label("Y",(-31/2,0),S);
label("A",(0,7),N);
label("X",(17/2,7),N);
label("Z",(-7/2,7/2),N);
[/asy]<spanclass=′latex−bold′>(A)</span>317<spanclass=′latex−bold′>(B)</span>217<spanclass=′latex−bold′>(C)</span>9<spanclass=′latex−bold′>(D)</span>451<spanclass=′latex−bold′>(E)</span>12