Why isn't this a right angle?

they aren't right angles, and they aren't perfectly square. Get a protractor.

Okay, thank you for your response.

Yes, I understand they're not right angles, but I am asking WHY they are not right angles. Could you please explain in more detail? I have never learned about right angles before. This is a sincere question.

I will try to get a protractor. Will Khan Academy teach me how to use one in higher grades?

Hi Jason,

That's correct, a rhombus does not have any right angles. In general, unless an angle is marked as a right angle (using the little square in the angle or a 90º marking), it is NOT safe to automatically assume that the angle is a right angle based on appearance alone.

A rhombus is a four-sided shape in which all sides are the same length and the opposite sides are parallel. A rhombus with right angles is a special instance, known as a square! However, unless the rhombus is also a square, it will not have right angles.

To help illustrate this, take a look at the following example:

If you draw lines connecting the opposite corners, these will always intersect at a right angle. The angle where these lines intersect (a right angle) is visibly different from the angles formed by the sides (not right angles). My example here is more obvious than the one you gave from the problem since this rhombus is more "stretched", but if you were to draw lines on the example that you gave, you should see that these two sets of angles look very different.

I hope this helps! Please let me know if there's anything else I can help clarify for you!

Maybe it’s because bent a little bit 

You don't really need a protractor. We know that all of the sides are of the same length (10cm) so we can just use a ruler to measure the diagonals and see if it's really a square. In a square, the diagonals would have the same length. A ruler is enough to show that the vertical diagonal here is shorter than the horizontal one and so you can be confident that it's definitely not a square.