A straight line, at an inclination of 45 degrees (from SW to NE) if the scales for the two axes are the same.
That is a function. x=y^2 is not a function.
You need it in the form f(x)= ... (whatever your equation happens to be). i.e get the equation in the form y=... Then swap the 'y' for 'f(x)'. Simple.
It depends on the domain of y. If that is restricted to non-negative values, then the answer is yes. But if y is allowed to be negative, then the answer is no because then there are two values of y...
4x - 10y = 11 Add 10y to each side: 4x = 11 + 10y Subtract 11 from each side: 4x - 11 = 10y Divide each side by 10: y = f(x) = 0.4x - 1.1