Symmetric Matrices & Quadratic Forms · Easy problem

A quadratic form $q(\mathbf x) = \mathbf x^{T}A\mathbf x$ on $\mathbb R^2$ is diagonalized by an orthogonal change of variables $\mathbf x = Q\mathbf y$, giving $q = 2y_1^2 - 5y_2^2$. Find the value of $q$ at the point whose new coordinates are $\mathbf y = (3, 1)$.