Take a line segment AB=7cm.
Now open compass app. more than half the line segment.
Put the pointer on point A and mark an arc upward as well as downward the line segment.
Similarily put the pointer on point B and mark arc upward and downward.
Name the point of intersections x and Y.
Now join XY which is the required perpendicular bisctor of line segment AB.
For verification, measure the two parts.
Each one will be equal to 3.5cm.