Question 29716
y - 2x = 0  ----(1)
y = 2x is of the form y=(m)x which represents a line 
through the origin with slope = 2
Let us now plot points after drawing the preliminary rectangular co-ordinate axes of reference(the x-axis andthe y axis) and marking the point O,the origin, giving values to x say:   
-4,-3,-2.5,-2,-1.5,-1,0,1,2,3,3.5,4,4.5,5,5.5,6 etc
we get the corresponding values of y given by
-8,-6,-5,-4,-3,-2,0,1,4,6,7,8,9,10,11,12, ...
we get points (-4,-8),(-3,-6),(-2.5,-5),(-2,-4),(-1.5,-3),(-1,-2),(0,0),(1,2),(2,4),(3,6),(3.5,7),(4,8),(4.5,9),(5,10),(5.5,11),(6,12)..
which when plotted and joined by a smooth line we get a line through the origin. If we measure the angle which this line makes with the positive side of the x-axis and consider the trigonometric function tangent of this angle which by definition is the slope of the line we get the value 2.