# Thread: How do I find the surface normal

1. ## How do I find the surface normal

This was on my lecture page, but I don't understand how they went from step 2 to step 3.

How does 2 & 4 produce -6? and 5 & -3 produce -20?

My homework problem is
A= [3,-5,7]
B= [-1,0,6]
and I'm having trouble trying to figure what to do. I've googled, but I think my terms might be confused.
All I know is that I need to obtain the cross product, and I don't even know what that is

2. Originally Posted by Keysle
This was on my lecture page, but I don't understand how they went from step 2 to step 3.

How does 2 & 4 produce -6? and 5 & -3 produce -20?

My homework problem is
A= [3,-5,7]
B= [-1,0,6]
and I'm having trouble trying to figure what to do. I've googled, but I think my terms might be confused.
All I know is that I need to obtain the cross product, and I don't even know what that is
Its evident you don't know how to evaluate a determinant. The deteminant for a 2 x 2 matrix is given by $\begin{vmatrix}a&b\\c&d\end{vmatrix} = ad-bc$.

So with this, you should be able to verify the values they get going from step 2 to step 3.

I hope this helps.