WHY must a heptagon add up to 900° (in pictures)

Because there are now 5 triangles in a heptagon and it must add up to $5times180°=900°$ .

There are 5 triangles that make up a heptagon therefore the internal angles equal 900 degrees

The heptagon can be made from 5 triangles.

Example 1

An equilateral heptagon will have 7 equal internal angles and would be

$900^circ/7=128.57^circ$

Each internal angle is 128.57°.

The interior angles of a heptagon equal 900 degrees

But the interior angles would still add up to 900°.