Confusion regarding surjective function I am a high school student, today our teacher taught us about onto function, explain only thing that I understand 
What I have understand -- that is two values of $x$ have same value of $y$ mean $y=1$ is a onto function but how find the function is onto for the given different equations.
Edit - i have one more doubt regarding the same function
what my teacher told me that for every value of x there is value in set y am if i am right suppose set x contain 4 element and set y contain 3 element one one function is not possible if my above statement is right please correct me if i am wrong
Is parabola is always a onto function 

Surjective is another word for onto. 
A function $$f:A\to B$$ is onto if for every $y\in B$ the exists at least one $x\in A$ such that $f(x)=y$
For example $f(x)=x^2$ is an onto function from $(-\infty, \infty) $ onto $[0, \infty)$
Because for every non negative real number $y$ we have $f(\sqrt x)=y$
