Three point charges are located at the vertices of an equilateral triangle. The charge at the top vertex of the triangle is given in the figure. The two charges $q$ at the bottom vertices of the triangle are equal. A fourth charge $-6C$ is placed below the triangle on its symmetry$-$axis, and experiences a zero net force from the other three charges, as shown in the figure below.

Find $q.$ The value of the Coulomb constant is $8\mathrm{.}98755{10}^{9}N*\frac{m^2}{C^2}.$

Answer in units of $C.$