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.
