What Does A - I_3 Signify in Linear Algebra?

[ssb]

Homework Statement

Lets say A = a matrix.

What is meant by the question $$A - I_3$$ ?

I don't understand what the I means and I am only given one matrix... a 3x3 matrix listed as A

 

[Dick]

I_3 is a 3x3 identity matrix - ones down the diagonal and zeros elsewhere.