(6x^4) + (15x^3)(y^2) + (3x^2)(y^3) is your answer
As we are looking at descending powers of x. Look at the powers of the x. In the first one (6x^4), the power is 4, and 4 is greater than the other numbers given (3 & 2). Therefore, the order given above is correct.
To combine like terms basically means that you combine variables that are the same and have same exponent together. think of x being apples and y being oranges, you can say you have 10 apples and 9 oranges but you cannot say you have 19 apple oranges. It is the same idea with combining like terms, variables are dofferent from one another as much as variables with different exponents are dofferent feom one another even if they are the same variable. As such we have: