Language: Sinclair Basic + ToBoS compiler. Running on ZX Spectrum clone. I wrote some computational program to help with my dad's research. It was successfully published so this was the first "useful" thing I ever did using my coding skills.

Added some references:

[1] The computer: https://en.wikipedia.org/wiki/Timex_Computer_2048

[2] Compiler: https://en.wikipedia.org/wiki/ToBoS-FP