Step 1:

The figurative number of a triangle is $\"\"$ .

The figurative number of a square is $\"\"$ .

Rewrite the expression of square as $\"\"$

The figurative number of a pentagon is $\"\"$ .

The figurative number of a hexagon is $\"\"$ .

Rewrite the expression of square as $\"\"$

The figurative number of a heptagon is $\"\"$ .

Observe the sequence of triangle, square, pentagon, hexagon and all.

The figurative number of a heptagon is $\"\"$ .

Solution :

The figurative number of a heptagon is $\"\"$ .