What is 39 decreased by 33 percent

The answer to your question is 6 percent because you have to subtract

39 is the base number, so it is the 100%
That decreased by 33 means you're looking for 67%

Easiest method is to divide the base number by 100, which gives you the 1%:
39/100 = 0.39

You now have the value of 1%, but you need 67%, so all you have to do is multiply 0.39 with 67, you get: 26.13