if T: V -> V has the property that T^2 has a non negative eigenvalue (lambda)^2, prove that at least one of (lambda) or -(lambda) is an eigenvalue for T.

The Hint that it gives is the equality:

T^2 - (lambda)^2 * I = (T + lambda * I)*(T - lambda * I)

where I is the identity matrix.

Any help?