Answer:
When multiplying polynomials, you have to multiply every term of the first polynomial with every term of the second, and then sum all terms.
So, if we multiply all terms of the first polynomial by 7x we have
28x3+28x2+42x
Similarly, if we multiply all terms of the first polynomial by 5 we have
20x2+20x+30
If we sum all the terms we have
28x3+28x2+42x+20x2+20x+30=28x3+48x2+62x+30
Let me know if this helps at all! :)