two triangles are said to be similar if:

1) corresponding angles are equal

2) the ratio of corresponding sides are equal

the second posulate is what justifies the statement that these triangles are similar.

we see that side rs of one triangle corresponds to side ab of the other, and side st of one triangle corresponds to side bc of the other.

if these triangles are similar, then, for example:

rs/st = ab/bc

now, rs/st = 5/6

and ab/bc = 10/12 = 5/6 = rs/st

thus the triangles are similar