Because f(x)=x, goes through origin. So x and y -intercept are one point (0,0).
Im guessing the largest side of the triangle, i mean cmon. its the largest.
For f(x) to be a function, the value of x should not be repeated. That is no x value can be more then once.
So the required table is

And for g(x) , not to be a function, we have to repeat x value that is
