# Thread: I forgot how to do this...help.

1. ## I forgot how to do this...help.

Prove that there is no line through (-1,2) that is tangent to the graph of f (x) = x^2+ 3x .

2. Hi Arturo_026

The line through (-1, 2) : y = mx + c

Subs. (-1, 2) to the equation gives c = 2 + m, so the equation of the line : y = mx + 2 + m

If the line is tanget to the curve, then they will have common point, so :
mx + 2 + m = x^2 + 3x, where m = f '(x) = 2x + 3

Then...

3. Hahaha.
It is pretty simple. The problem is that I was kinda nervous for the final that I was getting my memories black outs
But i just tookk it and I see it now.