Slope of a tangent From first principles

[Matt1234]

Homework Statement

# 23 http://img26.imageshack.us/img26/1008/questionsh.jpg

Homework Equations

m1 * m2 = -1

The Attempt at a Solution

i get -4 not sure what's wrong:

http://img4.imageshack.us/img4/6092/15608741.jpg

 

[Office_Shredder]

In general 2a*(-2a) = -4a²

The question is show that if you have a value of a where a²=1/2-a² (because this is where the two graphs intersect) then -4a²=-1

 

[Matt1234]

thats it forgot the POI, thank you.