In special relativity, light rays in Minkowski spacetime $\mathbb{R}^n$ travel along the light cone which, by definition, consists of all null directions associated with an indefinite quadratic form $q(x) = x^TKx$. Find and sketch a picture of the light cone when the coefficient matrix K is
a.) $\pmatrix{1&0\\0&-1}$
b.) $\pmatrix{1&2\\2&3}$
c.) $\pmatrix{1&0&0\\0&-1&0\\0&0&-1}$
I don't even know where to begin. This chapter deals with positive definites and it gives this question.