Let f(x)=ax+bf(x) = ax + bf(x)=ax+b. If f(f(x))=4x+9f(f(x)) = 4x + 9f(f(x))=4x+9, and a>0a > 0a>0, what are the values of aaa and bbb?
a=2,b=3a=2, b=3a=2,b=3
a=4,b=9a=4, b=9a=4,b=9
a=2,b=−3a=2, b=-3a=2,b=−3
a=4,b=3a=4, b=3a=4,b=3