A perpendicular bisector passes through a segment at 90° and cuts the segment in half.
If you look at the segment, it is a straight horizontal line that is 10 units long. The perpendicular bisector must be a straight vertical line for it to be at 90° to the segment and it also has to cut the segment into two 5 unit parts. That means it has to cross the segment at (10, 10).
A straight vertical line that passes through (10, 10) has the equation x = 10.