What are my mistakes when I plot these graphs?

Here’s my code:

documentclass[11pt,a4paper]{article}
usepackage[utf8]{inputenc}
usepackage{amsmath}
usepackage{amsfonts}
usepackage{amssymb}
usepackage{pgfplots}
pgfplotsset{width=5cm,compat=1.9}
usepgfplotslibrary{external}
tikzexternalize

begin{document}
begin{tikzpicture}
begin{axis}[
    axis lines = box,
    xlabel = $x$,
    ylabel = {$f(x)$},
]
%Below the red parabola is defined
addplot [
    domain=-10:10, 
    samples=100, 
    color=red,
    ]
{x^2 + 2*y^2 - 5 - 44/100};
addlegendentry{$x^2 + 2*y^2 - 5 - 44/100$}
%Here the blue parabloa is defined
addplot [
    domain=-10:10, 
    samples=100, 
    color=blue,
    ]
    {x^2 - x*y + y};
addlegendentry{x^2 - x*y + y}

end{axis}
end{tikzpicture}
end{document}

I want to plot graphs of these functions:
x^2 + 2y^2 – 5 – 44/100 = 0 and
x^2 – xy + y = 0.
Where are my mistakes?

documentclass[11pt,a4paper]{article}
usepackage{pgfplots}
pgfplotsset{width=12cm,compat=1.9}
usepgfplotslibrary{external}
tikzexternalize

begin{document}
begin{tikzpicture}
begin{axis}[
    axis lines = box,
    xlabel = $x$,
    ylabel = {$f(x)$},
]
% Below the red parabola is defined
addplot3 [
    domain=-10:10, 
    surf,
    %samples=50, 
    color=red,
    ]
{x^2 + 2*y^2 - 5 - 44/100};
addlegendentry{$x^2 + 2y^2 - 5.44$}
% Here the blue parabloa is defined
addplot3 [
    domain=-10:10, 
    %samples=50, 
    surf,
    color=blue,
    ]
    {x^2 - x*y + y};
addlegendentry{$x^2 - xy + y$}
end{axis}
end{tikzpicture}

end{document}