In your example we will have...
By doing the replacements we get :
21=t4+x 21=t3+x+x 21=t2+x+x+x 21=9+3x 3x=12 x=4
So we have t1=5 , t2=9 , t3=13 , t4=17 , t5=21 , t6=25 .... and we can find that t10=41
Working like before we find y=3 or y=-3
Now you can find t6.
Hope this is what you want...