Dear alkaeda,
I am glad you did not ask the question in mirpuri=the language of people from mirpur city in kashmir otherwise things would have been difficult. Just joking.
You see alkaeda, before numeral symbols were invented like romans people used to write numbers with shapes for letters. I hope you are familar with roman numbers ie i ii iii iv v vi etc etc etc. Likewise Arabs used to write alif, baa, jeem, daal, ha instead of 1 2 3 4 etc etc. Alif=1 baa=2 jeem=3 etc etc etc to do their maths. Letters denote values from 1 to 10 to 100 to 1000. That is alif=1, baa=2, jeem=3, daal=4, ha=5, wow=6, zaa=7, hah=8, tua=9. Then ya=10, kaaf=20, laam=30, meem=40, noon=50, seen=60, ain=70, faa=80 and suad=90. Then we have qaaf=100, ra=200, sheen=300, ta=400, tha=500, kha=600, dha=700, dhuad=800, zua=900 and ghain=1000.
Likewise, it was common to write a checksum of a word, sentence or the paragraph or even whole story or subject to prevent mistaken readings when it came to important documents. So muslims used to do the same for quranic text. This helped them read words correctly because in those days some letter shapes were not distinguishable ie there were no NUQAAT=dots in, above or below Arabic letters eg baa, taa, thaa, nun, yaa were all using a common symbol and so were, JEEM, HAA, KHA or FAA and QAAF etc etc etc. So while writing BISMILLA HIRRAH MAA NIRRA HEEM, they would also write the checksum for the sentence as well so that it could not be misread as something else ie the sum of alphanumeric value that is, they added value of each letter in the bismillah and wrote the sum of it which adds up to be 786. Later people started using 786 on its own which is incorrect use of it because it does not serve its purpose. Just checksum on its own does not serve its purpose because other words or sentences could also have the very same checksum eg ADAM=adam and MADA=female would have the very same aphanumerical value eg 1+4+40=40+4+1. However, if you had alif+daal+meem=adam=1+4+40=45=meem daal with its checksum, you would not read it as something other than adam.
The alphanumeric concept predates islam. The idea was around even in the time of torah. I hope you understand the explanation and good luck with it.
regards and all the best.