Put the line 2x-4y into slope intercept form

In order to make a graph, you need an equation.

  ' 2x - 4y ' is not an equation.  Without an equals sign (=)
it's just an expression that stands for a number.

There's nothing in the question that implies a slope,
or an intercept, or even a graph.

She will need 5/8 bags for each square yard.