Final answer:
When adding the polynomials (3x+7) and (7x-5), we combine like terms to find that the leading coefficient is 10. The completed addition results in the polynomial 10x+2.
Step-by-step explanation:
When adding the two polynomials (3x+7) and (7x-5), we combine like terms. To find the leading coefficient, we add the coefficients of the terms with the same degree. In this case, we add the coefficients of the x terms, which are 3 and 7.
3x + 7x gives us 10x. So, the leading coefficient is 10. The filled equation would be 10x + 2. Therefore, the question mark should be replaced with the number 10.