Similar triangles have equal angles. I mean, if two triangles
are similar, then the angles of one triangle are the same as
the angles of the other one.
The triangle in the question has angles of 20° and 40°.
The angles of every triangle add up to 180°, so we know
that the third angle has to be 120°
If another triangle is similar to this one, then the other one
also has angles of 20°, 40°, and 120°.