This is simply a math question. First of all you should find the middle of the two points Y1 (0,10) and X1 (20,0) which is (10,5). Now we should determine the slope of the line which is perpendicular on the line between X1 and Y1. That slope is equal to 20/10=2 Thus the equation of that line will become y-5=2*(x-10) and thus y=2x-15
To find the coordinates of Y2 we have to find the point where the x value is 0. that is on y=-15. Thus the Y2 coordinate is (0,-15). You can find the value for Y2 in the same way for other values than 20 and 10.