Construct 
of length 7.8 cm. From this, cut off 
of length 4.7 cm. Measure 
.
Steps of Construction-
1. Draw a line segment AB of length 7.8 cm using a ruler.
2. Take a measure of 4.7 cm from a ruler on compass and draw an arc on AB naming it as C.
3. Measure BC. BC = 3.1 cm
4