The corresponding sides of similar figures are always proportional.
Corresponding angles of similar figures are always congruent.
Hope this helps.
< coa = 150 ( given)
as ba and bc are tangents
so oc and oa is perpendicular to cb and ab respectively because in circle normal passes through centre
so in quadrilateral oabc
< coa + < oab + < ocb + < abc = 360
150 +90+90 + < abc = 360
330 + < abc = 360
< abc = 360 -330 = 30
so < abc = 30