Surjective function
Practical example: y= –2x+4. What is the pre-image of y=2? Solution: Here a= –2, i.e. a≠0 and the question is: For what x is y=2? We substitute y=2 into the function. We get x=1, i.e. y(1)=2. So the answer is: x=1 is the pre-image of y=2.