f(f(f(x)))=a+b( a+b( a+bx))
f(f(f(0)))=a+b(a+b(a))=27
f(f(f(1)))=a+b(a+b(a+b))=29
f(f(f(0)))=a+ba+b^2a=27
f(f(f(1)))=a+ba+ab^2+b^3=29
(1) a+ba+b^2a=27 -> f(f(f(1)))=(1)+b^3=29 so 27 + b^3 = 29 <=> b^3 = 29-27 <=> b^3 = 2 <=> b = \(\sqrt[3]{2}\)\(\)
after in f(f(f(1))) you are replacing and find the a!