How many different combinations can you make with 6 digits using the numbers 1 7 9 1 7 and 9?

This is really a question about permuations with repeated elements.

The formula for that is n!/a! b!... where a, b,.. are repeated

So we have 6 elelments with each repeated 2 times. so the number is.

6!/2!2!2!

This is 6x5x4x3x2x1/ 23

so we can cancel out the 4 and the 2.

6x5x3=90

So 90 different numbers.