There are many ways to find the answer. One of them is to find how long does it take to make one part and then divide 25.8 by the result:
5.4 ÷ 9 = 0.6 (that's how long does it take to make one part)
25.8 ÷ 0.6 = 43 (that's how many parts can be made in 25.8 hours)