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 poylnomial by 7x we have
[tex] 28x^3 + 28x^2 + 42x [/tex]
Similarly, if we multiply all terms of the first poylnomial by 5 we have
[tex] 20x^2 + 20x + 30 [/tex]
If we sum all the terms we have
[tex] 28x^3 + 28x^2 + 42x + 20x^2 + 20x + 30 = 28x^3 + 48x^2 + 62x + 30 [/tex]
