need to find distinct combination of 2 feilds

Hi,

i have 3 feilds in my dso brand ,serial number and machine type. for a brand there will be a number of combination of machine type and serial number. i need to find distinct machine type and serial number.

ex.

brand mt sn

ram 10 1A

ram 20 2b

ram 10 1b

ram 10 1A

result should b = 3.