Since you subtract 6 and you have to find the sixth term, you can do:
-6×5=-30 ( negative shows you're subtracting and since 75 is the first term, then you have to find the difference of the input/term 6-1, the sixth term minus the first)
75-30=45
You can check by actually continuing the pattern.
75,69,63,57,51,45,39......
