construct a triangle ABC whose perimeter is 12cm and sides are in the ratio 2:3:4 

let ratio constant x

2x+3x+4x= 12 cm 

9x=12

x= 12/9

x=1.3 approx.

2x= 2.6 cm

3x= 3.9 cm

4x= 5.2 cm

now, i think that u can construct a triangle with this info