$***$ Lines are perpendicular when slopes have product $=-1$:$,m_1=\frac{a}{b}$ and $m_2=-\frac{b}{a}$

Show that the tangent lines to $x^2-y^2=5$ and $9y^2+4x^2=72$ are perpendicular at $(-3,-2)$

Curves are called orthogonal if they are perpendicular at every point of intersection, as are these curves.