Step by step process
Convert first the mixed numbers to improper fractions before adding or subtracting, then simplify the answer.

1) 3 4/6 + 9/18 = 22/6 + 9/18 = 66+9/18 = 75/18 or 4 1/6

2) 10 - 6 5/6 = 10/1 - 41/6 = 60-41/6 = 19/6 or 3 1/6

3) 8 4/6 - 5 8/12 = 52/6 - 68/12 = 104-68/12 =36/12 or 3

4) 9 3/5 - 7 7/15 = 48/5 - 112/15 = 144-112/15 = 32/15 or 2 2/15

5) 2 7/8 + 3/8 = 23/8 + 3/8 = 23+3/8 = 26/8 = 13/4 or 3 1/4

6) 12 6/10 + 5 5/10 = 126/10 + 55/10 = 126+55/10 = 181/10 or 18 1/10

Congrats, you will have a perfect score