Answer:
Because 3(5x) is the same as 3 *5 * x
Explanation:
When a number is next to another in parantheses, it signifies multiplication. In this case, we already have 5 and x grouped together by a multiplication sign, so the entire thing is 5 * x. The statement below is key
Distribution in multiplication is only possible if a variable is grouped to another real number by a plus or minus sign.
So in this case, we have 3 * 5 * x, which is 15x. However, if we had 3(5 + x), we would have 15 (3 * 5) + 3x (3 * x).
Hopefully that makes sense!