What are the values of a that make A=0?

[~Sam~]

Homework Statement

if
A=
a 3 5
a -7 6
5 4 a

find all values of a that make A=0. Give your answer as a comma-separated list.

Homework Equations

Looks like the cofactor expansion would be used

The Attempt at a Solution

I'm really lost in this one.

 

[Hurkyl]

Looks like the cofactor expansion would be used

What did you get by using it?