If the line segments a and B are known, draw a line segment to make it equal to a-2b Figure, complete Just process

If the line segments a and B are known, draw a line segment to make it equal to a-2b Figure, complete Just process

First, draw a ray arbitrarily with a ruler;
Compare the length of a with a compass, put the tip on the end of the ray, draw a circle with the end of the pen, and cross the ray at point a;
Compare the length of B with a compass, put the tip on the end of the ray, draw a circle with the end of the pen, and cross the ray at point B;
Compare the length of B with a compass, put the tip on point B, draw a circle with the end of the pen, and cross the ray at point C (point C should be between B and a);
The length of line AC is a-2b