You can
put this solution on YOUR website!How do you mean by sutstitution method?
if you have two points A(x1,y1) and B(x2,y2), then the slope of the line going through this two points is
y2 - y1
------- = m
x2 - x2
to find the equation of the line going through this two points:
y - y1 = m(x - x1) which you will be able to reduce to an ecuation of the form
ux + vy + w = 0
If you want to find the slope of a tangent on a curve in a point on that curve you have to calculate the DERIVATIVE of the curve in that point. The value of it is equal to m.