We are given a triangle and told that line WY is a perpendicular bisector to side XZ. This line basically splits the triangle into two triangles: XYW and YZW.
Note that as both triangles have two pairs of sides with the same length and a congruent angle (angles XWY and YWZ are both 90°), both triangles are congruent. THat is, corresponding sides have the same length.
This leads to having sides XY and YZ to be congruent. So we have the equation

first, we add 5 on both sides. So we get

Finally, we divide both sides by 12. We get

Note that

so we have that
