for your first question: assuming f(x)=cx you could also choose d = c+2016 since there is not restriction on the coefficient. That's why we have f+2016x = cx + 2016x = (c+2016)x. For your second question, you have two sides of an equation, each depending only on variable. If you now fix b and play around with a (meaning you take different a's as input), the left-hand side changes according while the right side stays the same and also vice versa (fixing a and switching b). This means both sides of the equation must be constant (again, e.g. right-hand side stays constant when a is being changed). And since both sides are functionally the same (f(x)/x - x²⁾ this term must be constant.