Sort the list $A,L,G,O,R,$I,$T,H,M$ in alphabetical order by Selection sort

Solve the following recurrence relations using backward substution.

a$)x(n)=x(n-1)+n-1$ for $n>0,x(0)=0$

b$)x(n)=x(\frac{n}{3})+n$ for $n>1,x(1)=1$

c$)x(n)=2x(\frac{n}{2})+6$ for $n>1,x(1)=0$