If both triangles are similar, you need to find a scale factor from triangle ABC to triangle DEF. to do so, you must compare the sides you know (BC=9 and EF=90). How do you get from 9 to 90? You multiply 9 by 10. Now, since AC is similar to DF, and you know DF=360, you divide by your scale factor of 10 to get the length of AC, which is 36
