Question 257618
Hint: Starting with {{{f(x)=ax+b}}}, plug in {{{ax+b}}} for 'x' to get {{{f(f(x))=a(ax+b)+b}}}. Do so again to get {{{f(f(f(x)))=a(a(ax+b)+b)+b}}}. Now simplify to get {{{f(f(f(x)))=a^3x+a^2b+ab+b}}}. From here, just equate the two expressions and solve for 'a' and 'b'.