Steps of Construction
(i) Draw BC = 12 cm.
(ii) Construct ÐCBY = 90°.
(iii) From ray BY, cut-off line segment BD = 18 cm.
(iv) Join CD.
(v) Draw the perpendicular bisector of CD intersecting BD at A.
(vi) Join AC to obtain the required △ABC.
Justification
Since A lies on the perpendicular bisector of CD.
Therefore, AD = AC
Now, BD = BA + AD
⇒ BD = AB + AC
Hence, △ABC is the required triangle.