since the equation y = -2x, is in slope-intercept form already, we thus know it has an slope of -2, and any parallel line to that one, will have the same exact slope.
so we're looking for equation of a line whose slope is -2 and runs through 3,7.
[tex]\bf \begin{array}{ccccccccc}
&&x_1&&y_1\\
&&(~ 3 &,& 7~)
\end{array}
\\\\\\
% slope = m
slope = m\implies -2
\\\\\\
% point-slope intercept
\stackrel{\textit{point-slope form}}{y- y_1= m(x- x_1)}\implies y-7=-2(x-3)
\\\\\\
y-7=-2x+6\implies y=-2x+13[/tex]
