The number of rhombus in the following figure formed by the set of same equilateral triangles is

In the side-4 equilateral triangular grid, two adjacent unit triangles sharing an edge form a rhombus of side 1 (6 in each of the 3 orientations, giving $6 \times 3 = 18$), and two adjacent side-2 equilateral triangles sharing an edge form a rhombus of side 2 (1 in each of the 3 orientations, giving 3). The total number of rhombuses is $18 + 3 = 21$.