change in estimation

Rose purchased for $2o million on 1 may 2007 with estimated useful life of 6 years, estimated residual value is $ 1.4 million At 1 may 2010 the estimated residual value changed to $ 2.6 million. The change in the residual value hasnot been taken into account ./
solution 1: 2o million -1.4 million=18.6 multiply by 3 divided by 6 , depriciation 9.3
18.6 -9.3=9.3
at 1 may 2010
9.3-2.4=6.9 divided by 3 ,Depriciation 2.3
6.9 – 2.3=4.6
The change in depriciation 2.3 -3.1=0.8
IS that a right solution.