Answer:
3a^2 - 17ab - 28b^2
Explanation:
When we multiply 2 poly/binomials by each other, we multiply each element of the first nomial with each element of the second nomial and then add it all up in the end.
(a-7b)(3a+4b) = a(3a) + a(4b) -7b(3a) -7b(4b)
Now we simplify and combine like terms
3a^2 + 4ab - 21ab - 28b^2
3a^2 - 17ab - 28b^2
Hope this helps :)