Any linear function is injection (one-to-one).
How many times can a line cross the x-axis?
Solve ax+b=c for x. Let's call the solution x0.
Existence claim: There exist an x such that a * x + b = c. Proof: Make sure that x0 is well-defined. Here it means that one does not have to divide by 0 to calculate it. Substitute x0 for x and verify that both sides of the equation are equal.
Uniqueness claim: There is at most one x such that a * x + b = c. Proof: Assume there are two numbers x1 and x2 that satisfy this equation. Substitute x1 for x and then x2 for x; you'll get two equations. Subtract one from the other. You should get a * (x1 - x2) = 0. From here and the fact that a <> 0, conclude that x1 = x2.