If it has no slope then it will be a straight line parallel to the x axis.
When you are trying to graph an equation.
Yes
The graph of an equation can have a slope and an intercept. Sadly, "x-y-1" is not an equation, so it doesn't have any.
The slope-intercept form of the equation is y = mx + b, where m represents the slope and b represents the y-intercept. It is used to graph linear equations easily.
If you have anything that you're planning to graph, it's got to be an equation that has 'x' and 'y' in it. If you have an equation that has 'x' and 'y' in it and you're planning to graph the equation, then you've had enough elementary algebra to know how to solve the equation for 'y'. Do that first and bada bing, it'll be in slope/intercept form.