The thing to remember when you first graph an equation is,
y = mx +b
That equation is slope intercept form.
In slope intercept form, m is the slope of the equation.
y = 2x - 4 the slope is 2 (or 2/1) when graphing remember that it's always x/y
As you're graphing remember to go over 2 and up 1.
y = 2x - 4 the y intercept is -4. That's where you will start the graph on the y axis.
Hope that helps! :)