To draw DB perpendicular to AC means to draw perpendicular to AC from point B which is outside AC.
The steps of construction are given below:
Place the compass point on B.
Adjust compass width to beyond the line AC.
Draw two arcs across the line, creating two points.
From each of the two points, draw an arc above the line so they cross.
Draw a line from B to where the arcs intersect.
The new line is perpendicular to AC and passes through B.