36146

5+2=7
1+7=8
9+4=13
8+6=14
0+3=3

We wrote down all the digit numbers from 1-9, then started with the envelope number 7, and we thought of a possability which was 5+2=7. Then we crossed off the digits 5 and 2.

Next we knew we couldn't use the digits 5 and 2, so we thought to equil 8 we could use 1 and 7.

After that we chose the digits 9 and 4 to equil 13.

This is where it got tricky because we had already used most of the number cards. The numbers left were 8, 6, 3 and 0. So we took the biggest numbers as the number that we needed to make was 14. So we chose 8 and 6.

This bit was easy as we only had 2 numbers left, which were 0 and 3. We were quite lucky as what these number cards equiled, was the last number that we had to make.