5 times the sum of 3 times a number and -5

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5 times the sum of 3 times a number and -5

The phrase a number means an arbitrary variable, let's call it x:
x

3 times a number means we multiply x by 3:
3x

the sum of 3 times a number and -5 means we add -5 to 3x:
3x - 5

5 times the sum means we multiply 3x - 5 by 5:
5(3x - 5)