GPLv3 and LGPLv3 licenses are be compatible with the Apache v2 license, meaning you can incorporate Apache v2-licensed code into an LGPLv3 library. (Note that the reverse is not true, see https://www.apache.org/licenses/GPL-compatibility.html).
As for the way in which you used the Apache v2 licensed code, the Apache v2 does not apply too many restrictions. If you were to copy a handful of functions (or even lines from a function) into your LGPLv3-licensed code, you'd need to do the following:
- Ensure the applicable copyright, patent, trademark, and attribution notices are reproduced in your source (e.g. placed prominently in a comment block near the top of your header file).
- Note which code that license applies to. If you've copied functions, you could e.g. place a notice that identifies the functions by name. If you copied a block of code from within a function, you might delimit it with comments.
- Include a copy of the Apache License v2 in your code. Typically, I put these in a "Third-party licenses" folder in my documentation folder.
- If the code comes with a NOTICE file, reproduce this in its entirety.
Note that the other requirements of the Apache License are adequately covered by the LGPLv3 license itself.