The solution to this equation is x=4.6 because 19.32/4.2=4.6
10 animals would be going to the river
The product of 15 and

is

. This is a linear binomial.

is a cubic trinomial. In your original question, I'm not sure what the

means.
X^2 + 10X + 21
(X + 3)(X + 7)