Answer:
10.8 inches long
Explanation:
We can determine the width to length ration by dividing 10 inches by 12 inches:

For every 5 inches wide the paper is 6 inches in length. If the picture is reduced to 9 inches in width, we can calculate the length by taking the inverse of 5/6 which is 6/5 and multiplying by 9 inches:

the paper is 10.8 inches long