G'day Soggy Muffin
The answer to question 1 is the values of x that can be defined by the function. This is all real values.
For question 2 find the turning point of your parabola. Without using calculus this can be found by finding the midpoint of the 2 zeros
For question 3 rewrite the equation with a y instead of the f(x) then swap the x and y and solve for y.