Solve the system of equations by graphing? x+y=3, y=2x-15

To solve this you need to graph both first and second equation. The solution to this system is the point in which those 2 lines (each of the equation actually represents a line in x-y coordinate system) intersect. To get x and y coordinates of that dot you just need to make a projection of that dot on x axis and on y axis. Your graph looks like the one bellow. From it you can see that solution for system is:
x = 6
y = -3