Question 628437
This is simply subtraction of polynomials.

6b^2 - 4b + 3b^4 - (7b^3 + b^4 - 4b)

note: distribute the negative sign to the terms inside the parentheses to eliminate the grouping symbol, thus:

6b^2 - 4b + 3b^4 - 7b^3 - b^4 + 4b

note: combine similar terms in a decreasing order of exponent

3b^4 - b^4 - 7b^3 + 6b^2 - 4b + 4b

note: subtracting 3b^4 - b^4 gives us 2b^4 and adding - 4b + 4b gives us zero (0), thus we have:

2b^4 - 7b^3 + 6b^2 as the final answer