Answer: SAS Similarity theorem.
Explanation.
SAS stands for Side - Angle - Side.
SAS theorem states that if two triangles have corresponding angles that are congruent and corresponding sides with identical ratios, then the triangles are similar.
And that is what the figure shows:
1) The angle at M is the same for both triangles.
2) The ratios of the sides are: 2/3 and 4 /6. Given that 2/3 = 4/6 you have that the two corresponding sides have identical ratios.