If the straight line l is on the plane a, then any point on the straight line is on the plane a.

The intersection of straight line l and plane b is the common point of plane a and plane B.

The essence of finding the intersection of planes is to find two common points of planes.

Therefore, if two straight lines on the extension plane A and two straight lines on the plane B intersect at two points, the straight line connecting these two points is the intersection line.

The specific extension of which two lines needs to be judged according to the actual situation. The specific method is to look at several straight lines that make up plane A, and which extension lines obviously intersect plane B, so it is clear at a glance.

Pay attention to reading, in fact, the book is very detailed.