Question

So the question is "5(3x -5)" how do I know if the answer is 15x-25 or 15x+25?

Answer 1

Positive × positive = positive

Negative × positive = negative

negative × negative = positive.

So in your problem,
5 × -5 = -25

5 (3x - 5) =
15x - 25

Answer 2

Distribute. 

5(3x - 5)
5*3x - 5*5
15x - 25