Given :
AB = 6 cm, BC = 4 cm. and ∠B = 120°
To construct: Incircle of ΔABC
Steps of construction :

(1) Draw a line segment BC = 4 cm.
(2) Construct a ray making an angle of 120° at B of BC. Cut AB = 7 cm.
(3) Join A to C, and obtain ΔABC.
(4) Now bisects ∠ABC and ∠BCA that to intersect at O.
(5) Draw OD ⊥ AC and with O as centre and OD as radius draw a circle which touches three sides of ΔABC.
This is the in circle of ΔABC.