How many lines of symmetry does the figure have?
Lines of symmetry are the places where one-half of the image is a reflection of the other half. We want to know at how many places in this figure is one-half the reflection of the other half. If we fold our figure right down the middle, the right side will completely cover the left side.
What would happen if we try to fold it in half horizontally? It would look like this, because the bottom is not a reflection of the top. If we try to fold it at another vertical line, if we try to fold it here, we do not find one-half being the reflection of the other half.
For this irregular image, there’s only one way to fold it so that the halves line up perfectly, at the original blue line we drew. And so this figure has one line of symmetry.