can someone explain this please (3x)(5x)?

All this means is that you are multiplying 3x and 5x together. Firstly you multiple the numbers at the front, which will give you 15. Then you multiply the x's. Since there are two it is just x^2. So your answer is 15x^2. There is a simple rule to remember when multiplying and dividing indices and x's. 'When you multiply you add the indices, when you divide you subtract the same.' So another example following this rule could be (5x^2)(3x^4). Using the rule, you still get 15 out front, but adding the indices your final answer is 15x^6.
Hope this helped!

(3) (5) (x) (x)

15 x ²

simple just mulitply

3x * 5x = 15x^2

(3x)(5x)
= (3 * 5)(x * x)
= 15x^2