Does Slackware 15.0 come with OpenGL preinstalled, and if so, what version is it, and if not, how do I go about installing version 3.1?

The Mesa(3D) is the software which offer the OpenGL API under Slackware 15.0 and also there are the NVIDIA proprietary drivers, which has complementary support for particular NVIDIA graphics card.

However, depending on the graphics hardware support, the Mesa package of Slackware 15.0 gives you support from OpenGL 1.4 up to OpenGL 4.6

Practically, you will get maximum version of OpenGL supported by your hardware, then you will not get OpenGL 3.1 or superior, IF your graphics card has no support for it.

So, about what graphics card you talk? Please show the output of "lspci -v" and "inxi -G"

From /usr/lib64/pkgconfig/opengl.pc:

Name: opengl
Description: OpenGL (without GLX) headers and interface
Version: 4.5

Is that the information you're looking for? This is on an up to date installation of Slackware64-15.0.

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Thank you.

If you have an Intel or AMD GPU, then yes it does, and it will support the latest version of OpenGL that your GPU does.

If you have an NVidia GPU, then you need to install the proprietary NVidia drivers, which will support the latest version of OpenGL that your hardware does.

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