You haven't presented a whole system to solve. y = -1 is the equation of one graph, and this graph is the horizontal line y = -1. Graph this line.
Now supposing you have a 2nd linear equation, 3x - y = 5. Solve this for y:
y = -3x + 5. Graph this line on the same set of axes as you used for y = -1. Plot the y-intercept, which is (0,5). Starting at (0,5), move 1 unit to the right and then 3 units down. Connect the 2 points with a straight line.
You must now determine where the graphs of y = -1 and y = -3x + 5 intersect. Sometimes this is easy to do visually.
Suppose that you see the 2 graphs intersect. Determine the x- and y-coordinates of the point of intersection. THAT is the solution of your system of linear equations.
