This is a practice question in my book:
Find all linear functions $f$ such that $$(f \circ f)(x) = 4x+1.$$
Since linear functions are of the form $$ax + b$$
I do this: $$a(ax+b)+b\\ =a^2x+ab+b = 4x+1$$ If I solve the system $$a^2 = 4\\ ab+b = 1$$ I get two solutions: $$a = 2\\ b = \frac{1}{3}$$ However, I don't know what to do now. According to my book, the answer is that there are two functions: $$f_1=2x+\frac{1}{3}\\ f_2=-2x-1$$ I think I see the relation between my two solutions and the first function in the answer. But I don't know what happened with the second function in the answer. How did my book get that answer?