Answer :

Sure, let's multiply and simplify the given expression step-by-step.

We start with the expression:
[tex]\[ (5a + b)(7a + b) \][/tex]

To expand this expression, we use the distributive property, sometimes known as the FOIL method (First, Outside, Inside, Last):

1. First: Multiply the first terms in each binomial:
[tex]\[ 5a \cdot 7a = 35a^2 \][/tex]

2. Outside: Multiply the outer terms in the product:
[tex]\[ 5a \cdot b = 5ab \][/tex]

3. Inside: Multiply the inner terms in the product:
[tex]\[ b \cdot 7a = 7ab \][/tex]

4. Last: Multiply the last terms in each binomial:
[tex]\[ b \cdot b = b^2 \][/tex]

Now add all these products together:
[tex]\[ 35a^2 + 5ab + 7ab + b^2 \][/tex]

Combine the like terms [tex]\(5ab\)[/tex] and [tex]\(7ab\)[/tex] (which are the coefficients of [tex]\(ab\)[/tex]):
[tex]\[ 5ab + 7ab = 12ab \][/tex]

So, the simplified expression is:
[tex]\[ 35a^2 + 12ab + b^2 \][/tex]

Thus, the result of multiplying [tex]\((5a + b)(7a + b)\)[/tex] and simplifying is:
[tex]\[ 35a^2 + 12ab + b^2 \][/tex]