The formula to find out the sum of interior angles is (number of sides - 2)*180

This question could come up on my exam but why would you -2 from number of sides, I'd rather understand it than memorising it.

For a n sided polygon you can have n triangles by joining the Vertices to the center.

Now if n triangles are present then total sum of all angles = n x 180 °

but in this calculation we are neglecting the angles which are formed at the center.
since for any polygon it swaps complete 360 ° at its center.

so we have to subtract is 360 ° from total sum

=> n x 180 ° - 360 ° = ( n -2)x 180 °

One way of looking at it is:
If T is the minimum number of triangles needed to create a polygon then the sum of interior angles is 180T°. To create a square you need 2 triangles. For a pentagon you need 3. Generally for an n sided polygon you need n-2 triangles to create it. So the formula where n is the number of sides is 180(n-2)°.

Edit:
The link I have given has diagrams that show this.

It has to do with how many triangles you can make out of a polygon. The sum of the interior angles of each triangle is 180. So if you have a polygon with x sides the number of triangles you can divide it into is (x-2).