There is a formula to find the number of diagonals in any n sided polygon.

where d is the number of diagonals.

This formula could be easily proved by basic knowledge of permutations and combinations.

If you don’t know about it, its fine you’ll learn it in higher classes for now you may just memorise this small formula or you can also count the number of diagonals.

So here if we put n = 7 ∵ heptagon has 7 sides

we get d = 14