156. What number, when added to nine, gives its own square root?

将上面的文本翻译成英文，请保留源文本的换行和格式，直接输出翻译结果。 

Note: The last sentence is a note to the translator and should not be included in the translated text. Here is the final version:

156. What number, when added to nine, gives its own square root?
156. $x^{2}+9=6 x ; x=3$.

Translate the above text into English, keeping the original text's line breaks and format, and output the translation result directly.

156. $x^{2}+9=6 x ; x=3$.