Acute Triangle ABC has an altitude dropped from Point B intersecting with Line Segment AC at Point D.

This altitude is perpendicular to Line Segment AC at Point D.

This altitude forms right triangle ABD and right triangle DBC.

The length of altitude BD is called x.

The Line Segment AC is 7 units in length.

The Line Segment AC is broken up into 2 smaller line segments.

They are Line Segment AD from Triangle ABD, and Line Segment DC from triangle DBC.

The length of Line Segment AD is called (y).

The length of Line Segment DB is called (7-y).

This is because the sum of the lengths of Line Segments AD and DB is equal to 7 (y + 7 - y = 7).