Step by step
The question
Question: find x.
The exterior angles of a polygon are 90°, 120° and x.
1 Walk around the shape
Walk along the edge.
At a corner, carry the side on, dashed.
Then turn to the next side.
The turn is the exterior angle.
We turn at every corner.
All the way round, we end up facing the same way as at the start.
That is one full turn.
So the exterior angles add up to 360°.
So we add them all and make it 360: 90 + 120 + x = 360.
2 Group like terms
Terms are the parts between the + and − signs.
Like terms are the same kind: same letters, same powers.
Plain numbers go together.
90 + 120 = 210.
Now we have 210 + x = 360.
3 Remove the 210
We want x by itself.
The 210 is in the way.
To remove 210, we take away 210.
We do it on both sides.
Why both sides?
The = sign means both sides are the same amount.
If we change only one side, they are not the same any more.
210 and − 210 make zero.
Cross them out.
360 − 210 = 150.
So x = 150!
4 Check
Let's check: put x = 150 back and add the angles.
They add up to 360°.
So x = 150 is right!
Let's look back.
First: walk around the shape.
Then: group like terms.
Then: remove the 210.
Last: check.