B. (k+10f)(k-5f) you can tell in 2 ways. 1. if the second sign is neg then the signs in the factors are going to be one neg and one positive so that knocks out C and D. Then you look at the second term and since the 5 is positive then the 10 has to be positive so you end up with a positive 5kf.
The second way is to FOIL them out. When you check B you get

combine your like terms and you have your original expression of

Hope that helps
The distance depends on the time.
So now we need to find the constant of variation or the constant of proportionality.
To do so we must find y(the dependent variable) and x(the independent variable).
To find the constant(k) we must find y/x
Since y/x=k
Then 2.25/.75=k
k=3
B Is the answer ......... z
-12,3 ...................................
13.365 would be the price of that book on tuesday.