Sol :
Here
a=4
n=6
d=?
Length of tree after 6 years would be
a6 = a+(6-1)d
=> a6 = a+5d

and after 4 years would be
a4 = a+3d

given a6 = 1/5 (a4)+a4
=> a+5d = [1/5(a+3d)]+a+3d
=>5a+25d = 6(a+3d)
=> 7d = a
=> d=4/7