Answer
$\blacktriangle\ \blacktriangle\vartriangle\, \vartriangle\,\vartriangle\,\vartriangle$
Work Step by Step
A fraction represents $\frac{part}{whole}$. If 2 triangles represents $\frac{1}{3}$ of a whole diagram that means the 2 triangles are one of 3 sets of 2 triangles each in the diagram. "Of" tells us to multiply the number of sets by the number of triangles in the set. The diagram contains $3\times2=6$ triangles.