[sp]Denote the lengths of the sides of the triangle by $|\vec{QR}| = p$, $|\vec{RP}| = q$, $|\vec{PQ}| = r$, and the lengths of the altitudes by $|\vec{PA}| = a$, $|\vec{QB}| = b$, $|\vec{RC}| = c.$ Then the area of the triangle is $\frac12ap = \frac12bq = \frac12cr$, so that the ratios $\frac1a : \frac1b:\frac1c$ and $p:q:r$ are the same.
If $9\vec{PA}+4\vec{QB}+7\vec{RC}=0$, then the vectors $9\vec{PA}$, $4\vec{QB}$ and $7\vec{RC}$ form a triangle whose sides have lengths in the ratio $9a : 4b : 7c$. Also, each side of this triangle is in a direction perpendicular to the corresponding side of the triangle $PQR$ ($\vec{PA}$ is perpendicular to $\vec{BC}$ and so on), so the two triangles have the same angles and are therefore similar. Thus the lengths of their sides are in the same ratio, so that the ratios $9a : 4b : 7c$ and $\frac1a : \frac1b:\frac1c$ are the same. Therefore $9a^2 = 4b^2 = 7c^2$, or equivalently $p^2 = 9\lambda$, $q^2 = 4\lambda$, $r^2 = 7\lambda$ for some constant $\lambda.$
By the cosine rule in the triangle $PQR$, $\cos R = \dfrac{p^2 + q^2 - r^2}{2pq} = \dfrac{9+4-7}{2\sqrt9\sqrt4} = \dfrac6{12} = \dfrac12$ and hence $\angle R = 60^\circ.$[/sp]