Notice that there are 6 sums of 7 on the table provided. For example rolling a 1 and then a 6 would produce a sum of 7.
The reason the answer is 12 and not 6 is that you need to double it for the two dice. Let's say one die was green and the other was blue. The green die could be the 1 and the blue die could be the 6. That is one favorable outcome, but it could be switched. Maybe the green die was 6 and the blue die was 1.
Your answer is 15 over four z + 12. Using the distributive property, 3/4 • 5z equals 15/4z, which can't be simplified. Then 3/4 of 16 is 12. If you divide 16 by 4 (the denominator) and then multiply by 3 (the numerator) that yields your answer.