Question i can not use my 8*2 ram but i can use my old with new 8+4

comon guys i know someonu could help me i am new at replacing parts and it make me sad that it is no working
Download CPU-Z and get the motherboard model from the Mainboard tab. Also in CPU-Z, go to the SPD tab to get the Manufacturer, model and part number of the modules. On the memory tab you could also give us the current speed of the ram. You should probably take screenshots and upload.